Take a print out of the CEX price (£29) into your local Gamestation, they should match it and give you a £1 on top (my local does) so you'll at least get £30 for it that way.
it is a good game, you should persivere through the scenario mode, it is'nt half bad, and once you get a few mates round (or in my case, play it on live) it becomes very fun, provided your not completely awful at it

With regards to the receipt, I think peeops are just making sure you didnt get it from ShopTo and were trying to profiteer
Mike..