You don't need to do anything - you can't get your money back for an item that is showing as delivered.
48 hours after the tracking shows delivery, Ebay will release the payment from the buyer to you.
In the meantime, the buyer should open a case for 'item not received' - after they have checked around their property and asked the neighbours etc.
Do not make any promises to the buyer and don't refund them.
If there is evidence that the item was NOT delivered to the right address, Ebay 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)