[quote name='fwacce']While the OP shouldn't be doing this for ethical reasons, I must point out that you are incorrect in your analogy. If GameStop paid $40 for Ghost Recon, then they are out $40 to begin with, and they are hoping to sell it for $50 to make $10. If the OP takes his copy of PGR3 and exchanges it for Ghost Recon, then GameStop is still only out $40 in cash. Once they sell PGR3 for $50, then they have still made $10. This doesn't take into account that Ghose Recon is a better seller though and PGR3 may drop in price before it's sold, but that's another matter.
Got all that? So, the bottom line is it's still unethical to do it and that should be enough of a reason not to.[/QUOTE]
While that might be correct in theory, it's not in practice. They aren't profiting $10 in reality, when you factor in costs such as labor, rent, electricity, etc. If they pay $50, even in store credit for a game that should have only cost them $40, they have only broken even. There is no guarantee that they will sell a new game in return. Someone else would probably buy a game that the OP wanted eventually anyways, so it wasn't in their best interest to give them the credit.
Neither you or I know for sure, but what happens when MSRP drops? Does GS receive the difference from the manufacturer? I would have to think not. If the OP goes and trades in that game for hardware, that's even a bigger hole in GS's wallet because there is less margin on that. If the OP uses that credit on used items, it's an even bigger loss to GS. You may think it the opposite, but the used stuff is probably going to sell regardless. Which would GS rather do? Sell it to someone for cash, or for someone with store credit on a return?
I know someone can better explain the logistics of corporate accounting (I'm out of practice), but there is much more involved than what you posted.