Actually he's wrong. He doesn't have to sell it at all. Imagine it was a 52" Plasma Tv that had been marked up at 99p or better yet a Ferrari. If he sold that to you for the lower price he would be out of a job. A shop does not HAVE to sell you anything.
However if the price difference is negligble and above the wholesale price then most shops will honour the lower price in the name of good customer service.
I lived with 3 people studying law and I found the section on contracts and the sale of goods to be most informative.
Tis true, I stumbled accross a 42" HD TV on tinterweb for £49.99 (not £499.99 as it should have been) and promptly ordered 5.... Needless to say, the sale was not honoured based on the argument...
"Take me to court if you wish, but they will ask you if you really expected to buy a 42" HD TV for £50?"
If the transaction is complete (item in your hands, payment made with reciept) at an inaccurate price, then you're home free... And free to mock the cashier as you skip out of the store.
I've actually met Chris n shook his hand. This fact is still causing slight issues in my trouser dept....

Best post ever.