Nobody is ever obliged to sell you anything at the advertised price. For example, you could see something in a shop window with a '�20' price tag. When you ask to buy it, the shopkeeper can say "Sorry mate, it's now twenty million pounds". That's perfectly lawful. While you're ranting and raving at him, he can sell the item to the next customer for 1p. That's also perfectly lawful. (The only exception would be if he'd unlawfully discriminated against you because, say, of your race).
Exactly the same is true of items advertised on the internet. If the seller wants to change his mind about the low price he advertised (either because it was an error or simply because he decides he doesn't want to sell at that price any longer) he can refuse to sell you the item or demand a higher price for doing so.
However, if the vendor spots an error in his pricing after the completion of a sale, he'd have great difficulty in trying to get any more money from you (or getting you to return the item for a refund). The law basically states that a contract for sale is completed as soon as you've received the goods and the vendor has received payment. The contract for sale can't be varied because it no longer exists. Some traders (who've made massive errors in their pricing) have tried to use the provisions of the Theft Act 1968 to 'undo' the contract. That Act makes it illegal to benefit from someone else's error when, for example, a bank pays someone else's money into your account. However, the Act only makes it illegal to hang on to something (such as the balance of the payment which the vendor is claiming) when there's a duty upon you to return it. Contract law imposes no such duty upon a purchaser who's in receipt of a bargain and, to the best of my knowledge, all such court actions by vendors have met with failure.
Chris