Ebay don't get involved with deposits and, generally, any member asking for advice would be told not to pay a deposit.
This is between you and the seller.
if you believe it is worth it, then you should consider using Money Claim Online or taking legal advice.
There is nothing 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)