A key item to remember: Earth has an oxygen-rich atmosphere only because it has a huge interconnected web of organic life on it. Doesn't work the other way around. The presence of free oxygen in an atmosphere would be an instant signal that some form of metabolic life existed on a planet. Now, you can postulate a planet with life forms that do not produce oxygen as a by-product, but if the oxygen is there, you by definition can infer living things.
Not necessarily. You can have a planet with a vastly different chemical mixture than earth. True, oxygen tends to react and not stay as free oxygen, however if you have a planet with mostly oxidizing athmosphere not reducing athmosphere, the can still be free oygen around since there's nothing it can react with. This doesn't even have to be an all-oxygen athmosphere, for example the planet could contain a lot of halogen elements, which would oxidize available material and leave the oxygen free.
Anyway, another important point is that trade in space is pretty much impossible unless you have unbelievably cheap FTL travel. I'm talking about something like the wormholes in hamiltons commonwealth books.
Basically no material resource is ever worth enough to put it on a spaceship.
Simple stuff like elements (yes, even gold, even tritium), can be found in any amounts within the solar system. Before you go interstellar for that, you'd harvest the complete asteroid belt, all the lifeless planets (and their moons) heck, even the kuiper belt is just so much closer.
Technological artefacts are pretty much the same. The only machinery you'd ever transport at interstellar ranges are a starting set of factories that can make anything else you need. Just look at todays economics, car factories are already being built closer to customers to save on transporting the cars across the atlantic or pacific.
The same goes pretty much for any compound. Got this wondrous medical plant that cures cancer? Make one trip to ship over a few hundred tons of seeds or even a complete terrarium. Not multiple trips shipping the wonder drug. Got diamonds? Hell, we can make artificial diamonds today, and they're only distinguishable from natural ones because they're better! Got some strange animal that's the new must-have pet for the rich and dumb? Get a dozen and start breeding.
The only things that make any sense being transported across interstellar distances are information and people. (unless you have dirt cheap travel, see above. And remember, time is money. One-year trips are rarely cheap, unless the transport ships crew works for free)
Now, that being said, of course when that colony has just been established there would be quite a bit of transport. After all, all those pets and plants have to be shipped back to earth to start with the breeding and farming.
On the other hand, the colony has a whole planet to work with, they will be self-sufficient very very fast. Think of it in three stages. The first expedition start by building housings for themselves and means to grow food for many people. Then the second comes establishing a modern production base. (check out how long it takes to build a modern car factory for reference. Or even better a factory that builds industrial robots) And third, everyone who wants to migrate to the new planet can start to arrive.
A lot of this depends of course on your planet. I assumed a somewhat earth-like planet. There a countless scenarios though that can change the whole thing. It might very much earth-like, earth-like enough to have some nasty organic stuff to deal with. Alien viral diseases are unlikely enough to be pretty much impossible, however there's no reason a nasty flesh-eating alien bacterium couldn't exist. Or maybe in the blooming season all the pollen on the planet is poisonos for humans which they only find out three month in. And so on and so on. You could fill a book with nothing but the struggle against such a threat.
Another possibility is of course if the planet isn't earth like and has to be terraformed.
The easiest example would be of a planet with an oxygen atmospere and his own life that is incompatible with earth life. That'd just mean gradual killing off of the native life and planting of earth plants (and soil microbes, insects etc.) terraforming the planet piece-by-piece while there is still a working ecosystem that keeps the athmosphere breathable.
Or maybe there planet has a compatible atmosphere and climate and is simply dead, which means to terraform it you need to start a complete new ecosystem.
Or in the worst case it's totally uncompatible and colonists have to live in airtight buildings, or maybe city-sized dome structures until the atmosphere was changed. This however is again stupid at interstellar distances since there are planets and moons much closer so you can start by terraforming mars first.