In all honesty buy used on older games that are already marked down new. The savings can be amazing (I paid $17 for Oblivion). I would never sell them a game because they cheat the consumers and sell back at huge margins.
I support devs enough but sometimes it is great to leave a store with a cool game that didn't cost much= Gitaroo Man Lives for 2 bux :P
http://www.youtube.com/watch?v=F1gWECYYOSo
Please Watch/Share this video so it gets shown in Hollywood.