Wherehouse banned me from trading in games a couple days ago, after I traded in 6 copies of "Freestyle Street Soccer" for $17 each (among others). Those are especially nice to trade because every Blockbuster Video in the world is currently selling off about 20 of them for around $4.99 apiece, AND they're doing a buy 2 get 1 free deal right now. Comes out to 3 bucks per game, or $15 profit. It greatly helped me to pick up a DS.
Another great one to use is "Sneakers," which is TRU exclusive and is being cleared out for $5-10 at just about every store. They give $25 for that one!
There's nothing to worry about, really, but if you trade in the same game too many times they will tell you it isn't allowed (as the computer keeps track) and won't let you trade any more.