Those drop-shipper websites which are extremely numerous (and often haven't been running for very long) are quite guilty of this. They sometimes use this as a

Those drop-shipper websites which are extremely numerous (and often haven't been running for very long) are quite guilty of this. They sometimes use this as a way of "bait and switch" - we're sorry, we're out of stock, but if you want to use the credit for this item on other things...