Interstellar Trading

[Zoombie]

So, we all know the old hat of the space-trader, operating rather like the Dutch or English of the 17th and 18th centuries, traveling to other lands, finding rare goods and taking them home. Just transplant sailing to space and voila.

Well, that may work for a soft-science universe full of hand wavium faster than light travel. With FTL, you could have trade between solar systems quite easily and profitably. Or as easy and profitable as the specific FTL system i.

But what if you are trading in a slightly more crunchy hard-sci-fi universe? I've read a Deepness in the Sky and maybe I missed that passage, but what do the interstellar traders TRADE in a universe where it takes centuries to get from solar system to another?

The problem is, for me, the problem with the time scale. Lets say you get 100,000,000 tons of rare metals. You spend ungoldy amounts of energy to get to ramjet speed, then you spend ungoldy amounts of energy to decelerate, and then you arrive at a booming industrial planet to find, OH SNAP, they've had a nuclear war.

Or, worse, they've invented industrial nano-technology and don't need your stuff and laugh at your pitiful ramjet.

Now, most sane authors would at this point throw up their hands and ditch interstellar traders...OR they'd do what Vernor Vinge did and set up a universe where humans can only advance "so far" in terms of technology before they reach the limits of physical reality, meaning that no matter how long you spend between stars, planets will always need X, Y and Z, so you always have a constant to trade with.

But I've been kicking around a strange take on the interstellar trader thing.

What if traders don't trade simply for profit...but rather they trade to take a short cut. Relativity means that when they arrive at their target planet, technology will have advanced incredibly quickly or collapsed. So when they arrive at a HIGHER tech planet, they sell their cargo. If they arrive at a LOWER tech planet, they buy more cargo by trading some technical knowledge with the natives.

But what, I hear you cry, is the cargo?

Art!

Media! Music! Video games and books and paintings and murals and histories and culture. All the stuff that sentient beings create that is unique to that era, that technology, and that culture creates.

So, lets say we have Trader Family A get on their ramjet, their cargo hold full of paintings, music (on multiple kinds of format), and books. Loads of books, in all kinds of formats from holographic novels to stone tablets. They accelerate out of their system, journy for years, arrive at a distant world and find that this planet has collapsed back into savagry. So they simply use their fantastic space technology to wow the natives into selling some fine paintings or ornate creations or religious idols.

They travel on and find a decadent nanotech civilization and start hawking the culture they've found. Some is so far removed from the local peoples that its ignored, but enough "catches on" that the traders manage to make tons of money on the IP rights. They buy advanced technology, spruce up their ship, grab more culture and artforms from this civilization, and travel on, hoping to find a more advanced civilization in the starry horizons.

What do you guys think?

[Liosse de Velishaf]

If I remember rightly, Vinge's traders dealt more in knowledge than in physical goods.

I remember Charlie Stross' Singularity Sky had a race who travelled around giving out products in exchange for cultural material.

[Zoombie]

Ah yes, Singularity Sky was inspiring me.

The rain of telephones was an awesome opening to the book. I never got very far PAST that rain of telephones, but I'm...really lazy.

[Xelebes]

Most of the space trading would be done with elemental minerals (carbon, iron, copper, titanium, plutonium) where any system scarce of these resources would trade for some other element that is plentiful. I don't see much use for the trading of widgets/technology or food. Quaternary goods would be obvious.

[Lhun]

Well, that may work for a soft-science universe full of hand wavium faster than light travel. With FTL, you could have trade between solar systems quite easily and profitably. Or as easy and profitable as the specific FTL system i.Besides dirt cheap instellal travel(tm) what's also necessary to make trade of the kind of goods we're used to seeing on cargo ships a possibility, is pretty much an FTL drive that has no cost associated with mass.
But what if you are trading in a slightly more crunchy hard-sci-fi universe? I've read a Deepness in the Sky and maybe I missed that passage, but what do the interstellar traders TRADE in a universe where it takes centuries to get from solar system to another?In such a universe you pretty much don't trade.The problem is, for me, the problem with the time scale. Lets say you get 100,000,000 tons of rare metals. You spend ungoldy amounts of energy to get to ramjet speed, then you spend ungoldy amounts of energy to decelerate, and then you arrive at a booming industrial planet to find, OH SNAP, they've had a nuclear war.Timescale is one problem, the bigger problem is the sheer cost. You'll always be able to set up a production facility in system that can create the goods requiring far less energy than the trader takes to transport them. Raw materials are a non-issue.But what, I hear you cry, is the cargo?

Art!

Media! Music! Video games and books and paintings and murals and histories and culture. All the stuff that sentient beings create that is unique to that era, that technology, and that culture creates.Yes that is one of the pretty much only two kinds of cargo that are sensible to transport over interstellar ranges. The first is complete sets of manufacturing equipment, obviously necessary to kickstart a colony's industrial capacity to the point where they can develop it on their own.
Data could be traded much cheaper via simple broadcast, instead of STL ships. Now, a data trader makes sense when it's possible to have relatively cheap and fast small FTL ships, but STL, there's hardly a point. Even then, there's the question of whether there'd actually be any kind of trade, since getting any kind of payment is rather hard. If there are multiple highly developed planets specializing in different technological areas, it's possible. If there's earth and a lot of underdeveloped colonies, earth isn't going to be able to get any kind of payback.
Now, the second type of cargo is indeed art, or the like. Pretty much everything with a percieved intrinsic value people will pay for. All objects people will pay for to get an original from planet X instead of a (much cheaper) local reproduction.
Patents and the like are not something i'd expect to see at all. Intellectual property rights the likes of which we have right now are not exactly the natural state of things, and only work under quite specific circumstances.

[Zoombie]

Well, remember, the traders are not simply trading to trade. They're also making a HOME out of these spaceships, and using the fact that they will be visiting technologically distinct places to short circuit the general sweep of history. By serving on a ramjet trader, they can visit worlds that will have more advanced technology than the world they left behind, meaning that a good trading run could get someone nifty, nigh-magical new technology like advanced cybernetics, nanotechnology, genetic modifications, and other stuff they didn't have at home.

Plus, there is the freedom that ramjets bring. You're not just stuck on one planet, but rather your home is the stars. I like to think that there will be people who'd be drawn to that life.

So, remember, its not traditional "profit" these traders are after. Its more of a...life style.

If that makes sense.

[Lhun]

You have to fund your lifestyle someway. The problem with trading anything but art or luxury goods (whatever falls under "unique") is not a problem of small profit margins, but a problem of a huge cost of transporting freight over interstellar distances. It's just not going to happen that any planet can produce goods or raw materials so cheap that any other would reasonably buy them. It's much cheaper to simply get a download of the schematics, or that and a download of the schematics for the production facilities, and then build the stuff in-system. The king of trade we see today only works because there is a huge differential in production costs between, say, China and Europe, and the costs of transporting goods are extremely low.
Heck, over interstellar distances, it's quite possible you wouldn't even get a common currency going, since contact between star systems is so difficult and rather unnecessary. After all, what's some barkeeper on Alpha Centauri I want with credits backed by your Earth Central Bank? It's not like he's ever going to get to earth to spend them.

[GeorgeK]

I always thought the more logical cargo would be colonists.

[Smiling Ted]

But what if you are trading in a slightly more crunchy hard-sci-fi universe? I've read a Deepness in the Sky and maybe I missed that passage, but what do the interstellar traders TRADE in a universe where it takes centuries to get from solar system to another?


But what, I hear you cry, is the cargo?

Art!



The biggest problem is the insane variability of the art market. For centuries it was possible to buy Dogon masks and Pueblo pottery for a song, because they were considered too "primitive" to be art. The Parthenon was used as an ammo dump and the Mausoleum was demolished by Crusaders to build a fortress. Artistic beauty is in the eye of the beholder - but everyone wants to live longer, so a new medical technology would be a much safer bet.

Vinge's Qeng Ho trade in tech. They hold back certain technologies for trading purposes, while making core technologies freely available - so that rebuilding civilizations will want to trade for their more advanced items. And the fleets generally stay in a system for decades to help implement the technologies, becoming in-system powers while they're there. They're sort of a mobile industrial base.

Most of the space trading would be done with elemental minerals (carbon, iron, copper, titanium, plutonium) where any system scarce of these resources would trade for some other element that is plentiful. I don't see much use for the trading of widgets/technology or food. Quaternary goods would be obvious.

Any system with terrestrial planets would have enough elemental minerals of its own to make it a bad market for minerals shipped from another star system.

Technology and un-replicatable biological items (like a symbiont that scavenges toxins from your blood) seem the best bet for interstellar trade in an STL universe.

[TMA-1]

Unless exotic propulsions like Alcubierre or similar, or wormholes become possible, it will take a really long time to trade anything, even if you travel close to the speed of light. With industrial nanotech, you can build nearly anything, as long as you have the raw materials.

[Lhun]

I always thought the more logical cargo would be colonists.I'd rather hope colonist don't count as trade goods in the future. Though it can certainly make for an interesting (if dark) setting.

[Pthom]

Time dilation due to near-light speed travel prevents, as has been pointed out, ordinary trade between societies at either end of the trade route.

Faster-than-light travel is problematic in that few readers buy it anymore. Ie: it takes a deft hand on the part of the writer to make it work for the larger part of the SF audience.

But it seems to me that instantaneous travel between locations hasn't been exploited nearly enough. I'm thinking of a relatively old story by John Barnes (A Million Open Doors, 1992 (http://en.wikipedia.org/wiki/A_Million_Open_Doors)) where he posits a most viable trading culture based on the "springer" a type of interstellar transporter (http://en.wikipedia.org/wiki/Interstellar_teleporter). Other authors have used similar devices to good effect, but Barnes' novel talks directly about the trade ramifications of such a device. An important feature he discusses is how it takes generations to set them up, but that once set up, they enable rapid growth (and change) of the affected societies.

[Smiling Ted]

An interesting twist on the notion of trading knowledge in an STL environment - Niven has a story called The Fourth Profession. The knowledge is retail, not wholesale, though - it is in the form of RNA pills.

[Xelebes]

Any system with terrestrial planets would have enough elemental minerals of its own to make it a bad market for minerals shipped from another star system.

So say a system rich in europium would not trade for holmium from a system that was rich in it?

[DaveK]

Trading in a universe in which the voyages between partners takes hundreds of years sounds difficult to me. Do the traders have a stasis system or is it a multi-generation voyage? In most cases I can imagine, the travelers are primarily explorers and the trade is mostly an opportunistic event.

Trade goods come in three classes: raw materials- iron, helium 3 or unobtainium, manufactured goods- power supplies, computer systems, nanotech, art works, or knowledge- scientific, political, or entertainment. Without FTL the first is probably cheaper locally (exception unobtainium) and the last via radio. With FTL all three depends on cost.

[sunandshadow]

Back-and-forth trading voyages don't seem viable when so much times passes on each journey, because the goal of trading is to take the money home with you so you can invest in estates, spouses, educating your children and finding them good mates, bribes to local politicians... basically after one long journey you won't have a society to go home to.

Alternatively you can have a ship which is like a tribe, a moving home. In that case they don't care about money, their goal would be to get whatever resources are needed to maintain their ship, as well as more cargo to trade for the next batch of resources necessary to maintain their ship. That's maybe a bit more feasible, people might be motivated to abandon their home culture to live nomadically like this if they didn't want to live under the laws of their home planet, or if their home planet was a slumhole.

As far as what to trade, at least for a high-tech group visiting a lower tech world, I'd suggest services - surgery, agricultural genetic engineering, and plans for small money-saving or military-power-increasing improvements on current technology. But, if you were selling this sort of thing to a planet, you could probably keep right on selling it for decades, maybe even centuries, building yourself a nice kingdom there - so why take the risk of sailing away again?

[defcon6000]

I always thought the more logical cargo would be colonists.
Interstellar slave trade? :eek:

I don't see it being very plausible that a planet would load up a ship with a bunch of goods representing their era and set sail for a few centuries in the hopes of coming into contact with another culture who wants those goods. Where's the profit in it? ...Assuming this is a capitalistic society. But it is a HUGE investment to make and also uncertain if you'll ever see that money back, maybe your great-great-great-great grandkids will. But since we're an hedonistic species, logically, we wold never make such an investment.

Now if it was for say some altruistic purposes, like some rich people wanted to donate a ship and goods to help stimulate a savage society on the other side of the universe, whom they've never seen nor hope to meet, then that's more plausible...although not the trading you're looking for.

[Liosse de Velishaf]

Back-and-forth trading voyages don't seem viable when so much times passes on each journey, because the goal of trading is to take the money home with you so you can invest in estates, spouses, educating your children and finding them good mates, bribes to local politicians... basically after one long journey you won't have a society to go home to.

Alternatively you can have a ship which is like a tribe, a moving home. In that case they don't care about money, their goal would be to get whatever resources are needed to maintain their ship, as well as more cargo to trade for the next batch of resources necessary to maintain their ship. That's maybe a bit more feasible, people might be motivated to abandon their home culture to live nomadically like this if they didn't want to live under the laws of their home planet, or if their home planet was a slumhole.

As far as what to trade, at least for a high-tech group visiting a lower tech world, I'd suggest services - surgery, agricultural genetic engineering, and plans for small money-saving or military-power-increasing improvements on current technology. But, if you were selling this sort of thing to a planet, you could probably keep right on selling it for decades, maybe even centuries, building yourself a nice kingdom there - so why take the risk of sailing away again?



That's why Vinge's Qeng Ho travel in large groups and don't live on planets. They also get the benefit of not collapsing like so many other soceities do.

[Smiling Ted]

So say a system rich in europium would not trade for holmium from a system that was rich in it?

In an STL universe - no, probably not. After all, it takes light 4.3 years to reach our nearest stellar neighbor, and that's a system without habitable planets. To reach the nearest star with habitable planets, light would take...who knows? We haven't found any habitable planets yet. Ten years, fifty, a century. And that's just for a light beam. For an STL ship, it would take much, much longer.

Now how long have we been using holmium? No more than the last sixty years, in nuclear control rods and magnetic flux concentrators. A civilization poor in holmium and europium would find other answers to its technological problems, or it would mine those materials from other planets or asteroids within its solar system, rather than wait for a ship to arrive with a load of ore fifty years after the fact. Would GE wait half a century for holmium for its control rods? Or would it find a different material to moderate its reactors first, thus making those rare earths useless when they finally arrived?

Technologies evolve to take advantage of the materials at hand; compared to the concentrated value of a new technology, even the most precious raw material is no more than bulk cargo - not worth the expense of transport.

[Xelebes]

In an STL universe - no, probably not. After all, it takes light 4.3 years to reach our nearest stellar neighbor, and that's a system without habitable planets. To reach the nearest star with habitable planets, light would take...who knows? We haven't found any habitable planets yet. Ten years, fifty, a century. And that's just for a light beam. For an STL ship, it would take much, much longer.

Now how long have we been using holmium? No more than the last sixty years, in nuclear control rods and magnetic flux concentrators. A civilization poor in holmium and europium would find other answers to its technological problems, or it would mine those materials from other planets or asteroids within its solar system, rather than wait for a ship to arrive with a load of ore fifty years after the fact. Would GE wait half a century for holmium for its control rods? Or would it find a different material to moderate its reactors first, thus making those rare earths useless when they finally arrived?

Technologies evolve to take advantage of the materials at hand; compared to the concentrated value of a new technology, even the most precious raw material is no more than bulk cargo - not worth the expense of transport.

But Once the shipments start arriving, they will have the holmium to do what they want. It's of course all about keeping a steady supply going. If it's easier to find holmium in a holmium-rich system than to mine it, they're going to take the offer.

To put in a comparable analogy, the spice trade had shipments that took several weeks to a couple months either over land or over sea. At first, the Europeans accepted that the spices were rare and they could find substitutes - namely herbs, honey and garlic. But they still accepted the spice when it came in and it came regularly enough that they used it satisfyingly.

[pdknz]

Selling art is basically selling ideas. A painting can easily be reduced to digital information and transmitted at light speed, so why bother to actually haul around the paint and canvas? Likewise most art and knowledge. IMO, the point of interstellar travel won't be in hauling freight between inhabited planets, but colonizing new ones. Once you have the basics--DNA patterns for plants and animals, people and technology to generate a new civilization, it's just a matter of applying known principles to new places. Need to develop plankton? Download their genetic code and set up the DNA sequencer. Need titanium? Find a substitute. Need something really basic like iron or water? You are probably in the wrong place, and should have gone over there instead. Want to read the New York Times--it's a lot faster and cheaper to get it over the internet than to mail it, so do it that way.

It seems to me that it's actually a more interesting question about how to manage an interplanetary civilization. To live in the asteroid belt (I'm working on an idea for this, so it's well in mind), you need thrust or delta-V to move around, plus food, water, oxygen, pressure, mass (basically someplace to be), and other stuff like heating and cooling. A lot of that, maybe all of it can be produced off planet, but not always in a particular place. Hence the metal mining whole damned thing among the asteriods, the gas mining scoops, the farm satellites or worlds, and in between, the traders. Somebody has to move the rest of the stuff around, so there would be a trade of long haul truckers, taxi drivers, and suchlike. Fuel for the ships should be basically reaction mass for use in fusion powered engines--start with hydrogen and rock dust, wave the pen carefully, and you're there. Forming asteroids seems emminently doable with parabolic reflectors and a bit of applied physics.

I really think that if you want to do the interstellar trader thing, you absolutely need a cheap FTL drive, and I mean a really damned good one. We might get it someday. There seem to be more clues and cracks in the known science all the time.

Doc Smith did it with zero inertia--just a simple and elegant little cheat on present day physics. Buck Rogers and some others supposed a way to control or focus gravity. Once again, not too far away, maybe. Time travel is another easy one--just cancel out the time you would otherwise spend on the trip, and bingo, you're there. Wormholes, multiple dimensions; shit, it's a wonder we haven't stumbled across three or four solutions already.

[Lhun]

If it's easier to find holmium in a holmium-rich system than to mine it, they're going to take the offer.This is the part that's just not going to happen. Unless you're trying to create some really massive stuff (like orbitals or a Ringworld) resources in a solar system are damn near infinite. Additionally, you don't actually use up elements, it's a question of how many you need to have in circulation. Our want&waste economy is only that way because recycling is currently more expensive than mining for many raw materials. There's no way the costs of mining + interstellar shipping can be lower than recycling or intrasolar mining + shipping. The amount of energy and time required to cross those distances is just too large.

[Xelebes]

This is the part that's just not going to happen. Unless you're trying to create some really massive stuff (like orbitals or a Ringworld) resources in a solar system are damn near infinite. Additionally, you don't actually use up elements, it's a question of how many you need to have in circulation. Our want&waste economy is only that way because recycling is currently more expensive than mining for many raw materials. There's no way the costs of mining + interstellar shipping can be lower than recycling or intrasolar mining + shipping. The amount of energy and time required to cross those distances is just too large.

As I said, time is not the problem. Once the link is made and regular shipments are made, time is not much of a concern. Especially in a system where the mining capacity can exhaust the rare-earth minerals in manner of millenia. Sure there is going to be some recycling but if they are going to advance and build the megastructures, build the mega-robots, there is going to need to be some trading going on.

If there is going to be interstellar travel or colonisation, there is bound to trade of some sort, raw, finished and intelligent property - no matter how much energy it costs. Trade is what binds the civilisations together.

[Bartholomew]

This is the part that's just not going to happen. Unless you're trying to create some really massive stuff (like orbitals or a Ringworld) resources in a solar system are damn near infinite. Additionally, you don't actually use up elements, it's a question of how many you need to have in circulation. Our want&waste economy is only that way because recycling is currently more expensive than mining for many raw materials. There's no way the costs of mining + interstellar shipping can be lower than recycling or intrasolar mining + shipping. The amount of energy and time required to cross those distances is just too large.

Unless of course the solar system's primary source of Mineral X is in a narrow band of intense gamma radiation.

Or its technically in another nation's or corporation's territory.

Or if any number of technical or political reasons render it untouchable.

Or if the primary site of the resource is considered sacred by the in-power theocracy.

The universe is a huge place.

[Zoombie]

I'm just making them intersteller nomads, cause I love shipboard societies. So axing the trading thing, beyond (obviously) buying and selling resources they can make or recycle on their own ship.

I'm thinking something like the Warhammer 40,000 ships, but...not a soft-science nightmare.

[Lhun]

As I said, time is not the problem. Once the link is made and regular shipments are made, time is not much of a concern.Time is a concern because time is money. Maintenance for the freighter, wages for the crew, lack of interest for capital bound up in the goods, it all adds up.
Especially in a system where the mining capacity can exhaust the rare-earth minerals in manner of millenia. Sure there is going to be some recycling but if they are going to advance and build the megastructures, build the mega-robots, there is going to need to be some trading going on.If you have the technology to build that stuff, you need materials in an amount that no other solar system will be willing to part with. You'd have to set up your mining operations in unowned solar systems to get that kind of volume going. I mean, if we're talking about exhausted resources, all uninhabitable planets and moons strip-mined, all asteroid belts cleared, the oort cloud freighted in-system and used up, and there's still a need for more material, we're not just talking about a few thousand or a few million tons here.If there is going to be interstellar travel or colonisation, there is bound to trade of some sort, raw, finished and intelligent property - no matter how much energy it costs. Trade is what binds the civilisations together.That is thinking backwards. Trade might be the result of economic conditions that allow for it to happen and be profitable. Civilisations bonding is not some kind of natural law. If the conditions don't favor it, it won't happen.
Interstellar trade of manufactured goods is never going to happen at STL. (with the exception of manufacturing industry of course) Producing in system is always cheaper, even if you actually have to import the infrastructure for that first.

[Lhun]

Unless of course the solar system's primary source of Mineral X is in a narrow band of intense gamma radiation.

Or its technically in another nation's or corporation's territory.

Or if any number of technical or political reasons render it untouchable.

Or if the primary site of the resource is considered sacred by the in-power theocracy.

The universe is a huge place.Yes, and that's the problem. It's so unbelievably huge that pretty much no local problem can ever hope to be the bigger hurdle.

[Albedo]

Selling art is basically selling ideas. A painting can easily be reduced to digital information and transmitted at light speed, so why bother to actually haul around the paint and canvas?

I agree with most of what you said, but not this. You're comparing Jackson Pollock prints to actual Jackson Pollocks. Guess which costs more? Fine art is largely tangible, permanent objects with dollar value in exclusivity. Believe me, the status boost from having had that painting in your home hauled fifteen light years won't be slight.

[Xelebes]

That is thinking backwards. Trade might be the result of economic conditions that allow for it to happen and be profitable. Civilisations bonding is not some kind of natural law. If the conditions don't favor it, it won't happen.
Interstellar trade of manufactured goods is never going to happen at STL. (with the exception of manufacturing industry of course) Producing in system is always cheaper, even if you actually have to import the infrastructure for that first.

But think, if people are willing to travel those long distances to colonise, then they are willing to transport goods. It simply doesn't work in isolation unless they are willing to die off in their new homes.

[Lhun]

But think, if people are willing to travel those long distances to colonise, then they are willing to transport goods.No, that doesn't follow. If they costs are to high they won't be willing to. If the economic conditions don't support it, they won't be able to. There is nothing that means trade must be possible just because colonisation is possible.
It simply doesn't work in isolation unless they are willing to die off in their new homes.Any serious interstellar colonisation attempt has to be made with the intent to be completely independent upon establishment of the colony. Not everything has to arrive with the first landing, people and materials can arrive at later scheduled times, but communication and transport back and forth takes too long.

[Xelebes]

No, that doesn't follow. If they costs are to high they won't be willing to. If the economic conditions don't support it, they won't be able to. There is nothing that means trade must be possible just because colonisation is possible.
Any serious interstellar colonisation attempt has to be made with the intent to be completely independent upon establishment of the colony. Not everything has to arrive with the first landing, people and materials can arrive at later scheduled times, but communication and transport back and forth takes too long.

And the presumption that it takes too long is just silly. They are always going to try and communicate back and forth. Trading treaties can last millenia as long as things are traded back and forth with regularity. Energy is not an issue because if they have enough energy to colonise, they have enough energy to trade. Most of that energy is just on acceleration and deceleration.

Think of it this way, if we're going to harvest Saturn with current propulsion technology, we would still do it within a century, given we know iwhat we want to harvest. It take years to get there and take years to get back but we would still do it.

[Lhun]

And the presumption that it takes too long is just silly. They are always going to try and communicate back and forth.Communication just means pointing a high-powered signal laser the right way. Being able to talk doesn't mean being able to trade.
Time is, and will always be money. If we take a very favourable case of adjacent star systems only a few lightyears apart, we can assume the trip to take maybe two decades. (With some pretty good STL engines)
That alone will at least double the price of whatever is being transported, just because it is inert capital. And there is no exclusivity that will force one to pay the increased cost. Manufacturing and mining localy is always cheaper.
Art and entertainment, as Zoombie suggested, are the only things reasonable to trade STL over interstellar distances. Art because people will pay for percieved exclusivity, and entertainment because it can be traded via transmission.
Also, art and entertainment are pretty much the only things a colony can hope to pay the higher developed home planet with.
Trading treaties can last millenia as long as things are traded back and forth with regularity.And this is supported by what examples exactly?Energy is not an issue because if they have enough energy to colonise, they have enough energy to trade.No they don't. A colonist fleet doesn't have to turn a profit.Most of that energy is just on acceleration and deceleration.There is no "just". Energy is energy. The same amount of energy, in joules as well as work done by people, can be used to manufacture whatever you want locally.
Think of it this way, if we're going to harvest Saturn with current propulsion technology, we would still do it within a century, given we know iwhat we want to harvest. It take years to get there and take years to get back but we would still do it.We would do it if we want to get something from Saturn we can't get from Earth, or Mars. Noone sane would build a car plant on Saturn and ship back cars. And while there are raw materials that can be found in abundance on Saturn (being a gas giant) and not on earth (not being one) one can imagine mining operations on saturn in the future. But there is no such analogy for whole solar systems, or the colonists chose extremely badly. There's just no shortage of any raw material in a territory the size of a solar system that warrants importing more. Additionally, even if, for some unfathomable reason, a colony fleet picked a solar system with atypically few and monotonous stellar bodies to settle in, for trade to happen not only do they have to want to import raw materials of some kind, they also need to be able to offer raw materials of some kind which another solar system needs.

[Zoombie]

Again, this is why I've changed my "Traders" to "Travelers".

They're macrolife, essentially, and they only trade to get raw materials to repair their ships.

Now all I need is a plot and some conflict and I'm good to go.

[defcon6000]

Are they looking to conquer anything? That's always good conflict...except in the case of Battlefield Earth.

[FOTSGreg]

I'm thinking that rare (or even not so rare) spices might be a good trade item. There was a time when spices were more valuable than gold (some spices still are).

Salt ought to be available anywhere or at least some kind of salt substitute could be whipped up by anyone with a decent enough chemistry set, but spices such as pepper, paprika, mustard, garlic, onions, hops (better beer with Earth hops!), etc. might not be available easily on another planet (I'm thinking environmental issues mainly with "taste" or capability of growing the stuff).

[Bartholomew]

Yes, and that's the problem. It's so unbelievably huge that pretty much no local problem can ever hope to be the bigger hurdle.

Unless, you know, one of those problems was big enough.

[Lhun]

Unless, you know, one of those problems was big enough.Well, yes. And if my cat was big enough, she'd go hunting elephants.
Of course, everything is "theoretically" possible, for a certain take on "theoretically". It's not what we usually call possible, and not in the least plausible. If you have a good idea for a book about interstellar trade, it's imo better to just go with cheap interstellar FTL then. You'll likely get less people complaining. It's like the superman telephone booth thing.