Various stores try to rip me off too. Cheap games in place of pricey games I'm trading in. You need to study the keypad while they scan everything in. Don't walk away to shop, while they scan your pile. After you sign, good luck.
I was just about to say the say thing. I also write down all of the values on a piece of paper to make sure I'm getting the right values for each game and the total I'm suppose to receive.