Goozex is the easiest; it does it all for you. You can get some real bargains when people don't know the value of their own stuff, but you can't barter. Also, you have to wait FOREVER for some games. If you want easy, go Goozex.
Gametz, despite some negative comments here, is in my opinion the best around; albeit the most time consuming. To be honest, Vinny has a point that there are some greedy people on there; but you're on a site that calls itself Cheap Ass for god's sake! Unlike Vinny said, the people almost always respond, but be sure to know the values of everything you're buying or selling; you know what you're selling, you get a good deal. You don't, you get ripped off and the other guy walks off a bit richer. The best, but the hardest.
CAG is good for trading, but let's be honst here; this isn't really a trading site. You won't get ripped off here, but you won't be trading like crazy either. If you just want to sell or buy a few games, this is a good strategy.
Bottom line: Goozex is simple but easy, Gametz is intricate but hard, and CAG is honest but small.