Buying online a product that might be out of stock

Most of online stores mention in their terms of service that they cannot guarantee that a product is in stock at the time of the customer's order. Which means that when shopping online, I may be paying for something that the seller does not physically possess yet.

What does Islam say about that? Am I allowed to purchase goods that are not available at the time of the transaction, but going to be restocked then shipped to me?