When I bought my Slim PS3 from GAME it came with inFamous which I already owned and I wanted Tekken 6 instead. Both were the same price but they wouldn't let me do a straight swap. So I bought the inFamous bundle and then traded in that game, but despite it being sealed and never having even left the shop, they would only give me the standard pre-owned price for it. Scamming bastards.
There is an issue with VAT I believe. Technically, they would have had to break the seal of the game and sell it as pre owned, as it had already been sold to you. I've taken sealed games before, and they asked that I open them. I've also been in a few times and been persuaded into taking a used copy of a game, for the guy at the till to say its brand new and all the codes etc are in there....odd.
I still have £17 in reward points to use after my card was 'disabled', they are pretty poor for customer services as its a repeat issue that hasn't been resolved. At some point, I'll pick up a new game for £23, I just need to decide which one, and when.