Ebay won't do anything.
It is really important that sellers know the process of returns and how the Money Back Guarantee works.
Sellers are not expected to refund until the item has been returned.
If you do, all you can do is contact the buyer and offer to pay postage for them to return the item.
Not to labour the point, but other people do read this forum and may learn from your error.
Imagine you bought an item from a dress shop on the High Street which proved to be faulty or the wrong size. Would you expect to be able to go to the shop, complain and leave with a full refund without returning the dress?
Of course not. Ebay works on a similar basis - no return, no refund.
"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)