HAH! I worked at GameStop for a dark point of my life during college. The way that we absolutely fucked customers over was a perk to the shitty pay and retarded customers/co-workers I had to deal with. I would return it and get a sealed copy. Try returning it to a different GameStop if they don't have a copy at the one you went to. It is standard practice at GameStop to give you the proverbial finger whenever possible. Hell, the employees barely check the used games that get traded in. I wouldn't be surprised if 20% of them didn't work.
Anyway - just return it for a sealed copy. I never buy the gutted copy, and with good reasons that have been stated.