No offense, but that's why you don't trade in 12 copies of the same game to one store. I used to get questionable looks when I'd have 2 of the same game, and that made me feel like I was pushing it. Trying to unload 12 is just asking for trouble. If you don't want to get caught, next time try to be a little more discreet.
Thing is that at the time they had a promotion going on that when you traded like 5 or more games you got like 50% extra credit or something crazy like that. Flipping them 1 or 2 at a time would result in a loss, flipping all at once meant as I said a $100-$200 profit and a $20 PSN card.