Not very much I am afraid.
There is nothing you can do through Ebay. They won't reverse the refund or let you send the buyer another invoice.
You can ask the buyer again to either return the boots or pay for 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)