I would offer to pay to have the item sent back.
If they are willing, the buyer can let you know the cost and you should be able to find a way to pay them.
The buyer can't open a return case for an item they did not buy so there is not anything you can do through Ebay.
"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)