I go out of my way to avoid buying a used game.  Buying a used game puts money in the pocket of the seller, and not the company that made the game.  Its kinda like pirating a music CD, you are getting a copy of the album, but the band isn't seeing a dime out of it.

Whenever Gamestop (or whatever game store) sells a used game, the only one that is profiting from it is Gamestop.  So if you want to support the company for making the game, then you should buy it new.  Or if you can't afford to buy it new, wait till it drops in price or save up money.