What you just experienced OP, is called Gamestop's NEWSED policy. Rather than actually spend money on spare cases to put copies of the games artwork into, Gamestop will open NEW/SEALED copies of games and put those cases/artwork/manuals on the shelves.
Why they still do this and the gaming public in general allows them to get away with such a shady tactic is beyond me. Although I would have to venture a guess that Joe Football(the same idiots you see EVERY year buying Madden right at release like it's a totally different game)and Suzy Soccermom(who just wants to get in and out with her 6 whiny brats from 5 different fathers) just don't give a shit enough to complain when they get an opened but 'new' copy of a game.
To me, once a game is opened, it is NO LONGER NEW. Try as they might, NO Gamestop employee has EVER convinced me otherwise.
You just need to step back and actually take the time to determine which is the better purchase next time. At least with their used/pre-owned shit you have a SEVEN day, no questions asked return policy.
I've abused the 7 day deal a bunch, I admit it, since there are some games you just want to try out.
Of course, as others have said, they WILL catch up with you so don't do it TOO often.