Your buyer needs to open an 'item not received' case.
As long as you used the Ebay Simple Delivery label, unless the tracking shows delivery, Ebay will deal directly with your buyer and will refund them.
"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)