you r paying for the reliability and quickness of service as well... i mean you can sell your games on ebay for a lot more money, and everyone knows that fact, but there's still no guarantee that you will actually get paid, there are scammers out there... also you can take your games there and get cash/store credit in minutes... ebay takes more time than that... like i said they are a pawn shop, and they are bound by state pawn shop laws so it governs a lot of what gamestop is allowed to do...
i would never suggest to trade in a game that you think u be playing in the future because it is not worth it... only trade in if you don't want it anymore... get something for nothing