Depends on what game it is really. Games like Oblivion, Mass Effect, even good ole' SW: Battlefront I keep around because I enjoy playing them (high replay value, which is generally why I by games in the first place).
Games that have little replay value, or games I genuinely don't like (here's lookin' at you Dragon Age...) go right to the local Game Stop. Perfect example of no replay value, at least to me, Prototype. Just because I didn't get all the 'coins' the first time, is no excuse to go through that ENTIRE DAMN GAME again. Hey, game devs; COINS/TOKENS/PACKAGES/WHATEVER DO NOT EQUAL REPLAY-ABILITY.
But yea, sell bad games, keep the good ones. Hope that answers your question OP!