Depends on the game.
If I liked the game, I'll keep it for sure. If I didn't like it and one of my friends would like to have it, I trade it. If I didn't like the game and know no one who likes it, I sell it. ^^"

For example, I still own EVERY SNES game I ever owned, except for one, which I TRY to trade on a website. But I wouldn't ever sell or trade the ones I own. I don't play them anymore (or at least: I'd like to, but I don't play games through twice... normally ^^"), but I think that maybe one day I'd like to play them again. Nostalgia and such. XD