Well, I've never worked at one of these flea markets, but I've certainly been to a few, and made some purchases at them.
Do you go to them? What are you looking for/what do you buy?
I do (obviously) go and I actually go whenever I reasonably get the chance (not that this is all that often). I'm always on the specific lookout for two things, both minor quirks of mine. One, I'm always looking for arrowheads (or spearheads, scrapers, etc. -- American Indian artifacts). I collect these, and although I prefer to find them on my own, I'll purchase them when I find ones that appear to be genuine and affordable. An upside to purchasing these is that people who find them (generally hunters, farmers, or kids) often have no idea of their actual value. They're not especially easy to look up (not like a coin, for instance), so they're often underpriced. I suspect that this is a reason why a lot of people go to these flea markets; they hope to discover an antique or some such whose value they are more familiar with than the seller would be. The second thing I look for is any unique chess set. These are much harder to find in an affordable price range -- but I'm always looking anyhow.
Beyond those things, I'll browse and generally enjoy looking at all the neat stuff. I've bought books (surprise, huh?) and artwork (you can sometimes get very good artwork pretty cheap at flea markets, if you're shopping for a picture, not the name on the picture), and I think a few random gadgets and gizmos.
What about the piece makes you buy it? Is it the aesthetics? The story behind the object? Functionality? A treasure hunt?
Probably more the treasure hunt factor, although the functionality comes into play for anything other than the arrowheads and chess sets. The story behind the item is meaningless to me (although I'll usually try to find out where the arrowheads were found, who found them, etc.), because the seller can be, and likely is, just making it up to sound neat.
How much are you willing to spend on that special conversation piece?
No more than I can convince myself that I can afford.
This is therefore a different amount at different times. Depending on how unique the piece is, I might try harder to convince myself to afford it. I try never to listen to the seller about how quickly this will probably disappear, and how I'll never again see anything like it for this cheap, cheap price of thirty percent off, just for me.
I really enjoy browsing flea markets, even if I leave without buying anything. I don't like shopping, but there is a different feel to wandering through the stalls/booths and looking at things you've never seen before, or thought you'd never see again. I wouldn't want to do it every weekend (though there are people, especially these antique treasure hunters, who do), but it's fun every once in a while. If you have any other questions, feel free to let me know. Hope this helps.