You don't seem to have any recent transactions.
If you had given more details it would be easier to advise you.
Generally, you have 30 days after the refund is made/case closed to start an appeal.
You can only appeal if the case closed without seller resolution. So, if Ebay refunded because you refused to engage with the case or customer. If you refunded voluntarily, you can't appeal.
If the buyer went for a chargeback through their payment provider, the advice would be different.
"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)