One of the oldest trading sites, GameTZ has one of the more effort-intensive systems we encountered. "Buyers" make a list of what they have/want, and the site matches them up with "sellers" who have it. However, at that point, it's up to the two people making the trade to barter between them until the deal is accepted, and then it's up to them to swap addresses and send the items.
That's a little more work than some of the other sites make you do, and finding a person who has the exact game you want AND wants the exact game you have can be tricky, even if the overall selection of games is better here than on any other site. But our bigger concern is that GameTZ also has the loosest controls on how to handle it when trades go wrong. You can definitely report bad traders, but you're still out of luck. Some of the other sites offer more security. This site relies upon community support rather than built-in failsafes.
It is cool that you can set up trades with an instant chat, though. Most other sites don't allow that. And if you're down with a little more leg work and a tad more risk, you've got a good chance of finding what you want and maybe making some friends as well.