You can use the link below to get a callback.
However, if the scenario suggested is correct, then you are unlikely to get anywhere.
If you had sent a label promptly for the bike to be returned, then refunded the buyer yourself, you would have your bike back and wouldn't owe Ebay any money.
Your only recourse now might be the small claims court but you would have to look properly into that before trying to make a case to get the bike back.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
"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)