I buy used games, but it's usually older games that are generally difficult to find new. Gamecube games, for example, are nigh impossible to find new anymore. Your best bet to find a copy of, say, Metroid Prime 2: Echoes or Digimon World 4 (both of which are good fun, IMO) is to go to GameStop or Bookmans (A used store chain in the southwestern US) and browse their used game sections. Even then, you'd have to check at least a few times a week, especially with Bookmans. Their stock shifts fairly quickly. Not only that, but new games are almost always $50 or more these days, and the lion's share of games that I like are, as I've said, older games that are hard to find new.
So yeah, I buy used games, but I have my reasons.