Was it posted in an impenetrable box? No? Well then it could have happened during transit.
You will not win this case. You will have to accept the return, or else eBay will rule in the buyer’s favour, and they will get their money back and get to keep the coat too.
When you receive the coat back, and if you’re still convinced the buyer has done the damage, then you can report the buyer and escalate the case. eBay may refund you the money. But as you cannot prove it wasn’t like that before posting (you could have sent a different coat to the one you photographed, for all eBay knows), or that the courier didn’t do it, then I can’t see that you will be successful. If the buyer has the original packaging, and that shows damage, ask for that to be returned with the coat, as you can then make a claim with the courier for damage during transit.