I try to buy only really good games- those that get about 9.0 or higher out of 10 in IGN or EGM. Ocassionally, I'll make a mistake and then sell an overhyped game. Also, I'll sometimes sell sports games to make room for the next version.
I sold some classic systems/games in my time and wish I did not. I like to collect the great games. I still have playing Goldeneye multiplayer with friends on my N64 and I imagine I'll do the same with Halo in years to come. Also, I like to play Legend of Zelda Ocarina of Time every other year to remind myself what perfect gameplay feels like.