You need to cancel the sale and refund the buyer.
Ebay will release the money to you after you make the refund although you may be out of pocket for a short time.
if you cancel saying the item is 'out of stock', you will get a defect on your account.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)