That would be for trading standards and alike to determine if the ad was a deliberate act to mislead customers (ie done deliberately to get customers to go to their shop but never actually going to sell at that price, or whether it's just a typo) if the decide it's the former, they will likely...
No they don't have to sell and the potential buyer doesn't have the right to buy it at that price, customer service may mean that the seller might be willing to sell it for that price but they are not under any legal obligation to meet that price.
The seller should withdraw then withdraw goods...
The shop would usually give the customer the deal at the wrong price, but it had nothing to do with being legally obligated to do so but they do it from a customer service point of view because they want to keep you as a customer so that you will continue to shop and spend there