All the time. I use the site called GameTZ to trade on. And man, so many of the people there are greedy.
I will look up values on eBay before offering a trade there, just to be sure I'm not being uneven. And more often than not, I'll get a counter-offer where they want like 5 of my games for 1 of there's. It's so frustrating.
I understand people want a deal, but when you're trading, you're not some profit-making dealer, you're just another gamer. So don't try to screw over your fellow gamer, just make a nice even trade. That's kind of the point of trading.
My BIGGEST pet-peeve with the people at that site, though, is when they just plain ignore your offer and never respond. There is a decline button. It's not that hard to click it. Drives me nuts!