I have seen that happen from time to time.
The policy when buying new games at Gamestamp is: Once the shrink wrap is gone, then the game is yours. You CANNOT return it. You have a 30 day window for an exact exchange, but that is it.
People buy it, get it home, and then they trade it in cause the manager isnt willing to work with the customer and just take the return.
Being the advocate of the customer that I am, I will just take the return and be done with it.