They really shouldn't be able to, considering the adverts are quite prominent in the shop. The worst my shop did was refuse to completely match a few prices, as the CEX trade price was more than they were selling the game for. In that instance, they just gave me the price they were selling it at (which I thought was decent, as they were making no money on that game).
I've been to my local CEX enough to know those who love any BS excuse they can think of to knock your price down.
I purposefully avoid them.