Like most long term and savvy sellers I build a small amount into my selling cost to cover the cost of a few refunds and it costs me nowhere near the total amount I collect, if it did I would have to increase that amount or stop selling if it was no longer cost effective. Look at it like shoplifting from a B&M store, no matter how many precautions you take it is going to happen that you take a loss at some time or another through dishonesty.

I doubt if ebay have done anything that is not covered in their terms and conditions but if a buyer has been refunded and not returned the goods or has returned them after inflicting damage or substituting something else you should go after the buyer independently of ebay, if it is of an amount significant enough to take the time.

Send them a message telling them that because they did ?????? you require them to deposit the sum of $?? into your bank account within 48 hours or you will be reporting them to the police through ACORN for obtaining goods by deception and (if a very high cost item) starting a small claims case in order to get your money back.