There is not anything you can do through Ebay.
The Money Back Guarantee has a process which protects the buyer and the seller.
Buyer starts a return.
Seller provides a label.
Buyer returns item.
Seller refunds.
There is no expectation that you will refund without getting the item back.
I always think, What would happen if I bought a dress from Marks & Spencer then went in a few days later and asked for my money back? They would ask me where the dress was! No dress, no refund.
Never refund until/unless you get your item back.
"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)