There's a good reason they are giving such a good price for RDR, it's selling for £50 used in the shop. This was in the shop in Edinburgh, and I was utterly amazed they were doing it for such a high price used, and the stock isn't even that bad now.
There is a reason for that.
When the game is really new, CEX won't have very many copies yet, so they get a high price (basic supply and demand). What explains why that price is more than buying a new copy elsewhere is that some people don't have the cash to buy the game new, but do have either the games to trade in, or existing credit notes to pay for a CEX one, and are willing to pay the premium to play the new release they couldn't get with cash.