It depends on whether a contract has been made between the two parties.
The retailer needs to accept the customer’s order for there to be a contract. If it hasn’t accepted the order it can withdraw the product from sale and cancel the order. Exactly where you stand will depend on the website’s terms and conditions and the wording of any e-mail sent to you when you placed the order.
Many websites say in their terms that an order is only accepted when the goods are dispatched. Any e-mail sent to the customer beforehand is simply an acknowledgement of receipt – as opposed to acceptance - of the order.
READ AND CONFIRM THE DETAILS OF THE EMAILS
But in some cases the wording of the e-mail will have accepted the customer’s order and the customer would generally be entitled to purchase the goods at the advertised price.
