davewil has pointed out the nub of the issue.

If you mean that the buyer has breached eBay policy, there are avenues for reporting that.

If you mean that the buyer hasn’t paid, there is an appropriate action to take.

If you mean the buyer is claiming the item hasn’t arrived, and you think that the buyer isn’t being truthful, only a tracking status of “delivered” can be your defence.

If you mean the buyer is claiming the item isn’t as described, you will almost certainly have to accept the return request and provide a return postage label, and refund upon receiving the item back.

What is the bad buyer behaviour in question?