As I understand it; the displayed price is "an invitation to treat". It does not mean that the seller has to sell you at that price. I assume that this is as true on the web as it is in a shop.
However, if you you receive an acceptance to your offer, (either verbally, or by e-mail), then a contract has been made. In these circumstances the seller is legally obliged to sell you the item at the agreed price.
I am not a lawyer, but have assisted in many law exams at a university, where this problem is an old favourite in Contract Law exams.