That's not entirely accurate, though. While they'll buy just about anything, if it sucks and they know they aren't going to move it, they'll buy it for a dollar or less. You won't see a game for cheaper than five bucks in that store, so using your example, they buy fifteen copies of a game for a dollar each, and sell six for five bucks a pop. Their expenses are $15 and their profits are $30, plus they've got seven extra copies of the game in inventory, which they can either sell through the online shop or keep in stock for the next time someone wants a copy.
Yeah but you also got to figure the more a game sucks the more copies that are going to come in to the store and the less they are going to sell of said game. I think the used game market is more driven by word of mouth than anything else so if the game has bad word of mouth the store isn't going to sell as many copies.