I don't trade in my games. I like to have a collection, and I like knowing that if I ever feel nostalgic or something, they're right there for me to play.
That's not to say I've never traded in games though. But the primary motives to trade in a game for me are that it has to be worthwhile financially (as in I'm almost getting a new game for trading in the old one), and that I'm pretty sure I'm never going to want to play that game again.
For those reasons, the only games I've traded in are Borderlands, Crackdown 2, and Army of Two: 40th day. I got bored to tears of Borderlands, and Crackdown 2 and 40th day felt fucking INFERIOR to their predecessors. Plus they're all co-op, so I knew I wasn't ever going to play them alone. The second I heard I could get $40+ dollars for trading them in, I was all over it.
Other then those disappointments though, I keep em.