I find that most used games at Gamestop still cost more than I'd want to spend, and they never come close to giving me enough credit for any game that I'd take in.  That's why I trade for/buy games that I want via Craigslist and Gametz.com (or even CheapAssGamer to some degree).  It's more of a pain in the ass to do it that way, but I can dictate the terms of the deal.