I always thought there was a subtle difference between goods priced up in a store, and advertised prices?
If an item has been advertised at a wrongly low price, they have to honour that price. But if it's just wrongly low in the store itself, then they don't.
But Rocko's right, a trader doesn't have to sell you anything, at any price, if they don't want to.
I'm sure I remember something about Tesco having some mis-priced dvd recorders on the shelf in one of their stores - £15 instead of £150, or something. A customer spotted it and went to buy one - Tesco said it was an error, but the bloke wasn't having any of it and started kicking off. So they sold it to him for £15 as "a goodwill gesture".
But then he went back to the shelf and loaded up a trolley with all the mis-priced stock of them, to buy them all at £15. Tesco then promptly advised him to push off.