I know this sounds extreme, but look into small claims court. I had this situation happen to me a week ago (although they did actually give me Stranglehold back ...the day before the TIV dropped like a rock), but they treated me with respect and didn't even start dropping psuedo-threats once they knew the game was missing. In fact, you might need to act quickly, because I'm almost 100% sure they have surveillance tape and you should be able to clearly see the faulty party.
My opinion? I think it's highly possible the CSR left the games on the counter and someone else may have ganked them. I've seen it happen time and time again.
ACT QUICKLY, the tape loops after so many days and be sure they allow YOU to watch as well.
Not just that, but you did sign over the games to them. They HAVE TO PROVE that you took them back or walked out with them. They can't reverse payment, they can however hold it or freeze it. Talk to the manager and if he don't listen, get his name and ask for his regional manager. Call his manager and corporate and they will fix this. BBV is desperate to get people trading in stuff and having people buy other stuff in the store. It doesn't matter that you traded in multiple copies of the same game. It was your property to begin with and you excersised your right to sell it to them. Flipping stuff is how business works. Just like BBV buys stuff at manufacturer's price and then sells it for $60 to you, you can do the same with your property. Trust me I've had run-ins with employees and managers like this before. One simple call to a distric manager or corporate will set things straight. Under no circumstances do they want to lose your business. Just say the magic words "I'll never buy/rent/trade from this company again, im goint to someone else" and they'll fold like a paper napkin.
By the way, I used to be a district manager for Funcoland way back in the day and corporate always said "make the customer happy even if they are in the wrong, as long as it keeps them buying here" Same was said to me when Gamestop bought us out. It's all about the benjamins.