It is horrible what you have experienced. Me personally if the buyer started saying about a damaged item before I dispatched it. I would have immediate cancelled that transaction and blocked the buyer. What you have said does sound like a scam and that was his intent to either bully you into a full refund or send back a faulty item with you paying for it.
Firstly you should block the buyer now to prevent potential other issues, even if you do consider to selling in the future, at least he is blocked for life. You can do so by copying his username in here.
https://www.ebay.co.uk/bmgt/BuyerBlock
Secondly, If you have already started a return. Ignore any messages sent to "My Messages" and go through the message system in the return only. When you appeal, eBay CS will not make a decision from "My Messages" unless you call up CS to point it out.
Unfortunately for you, you can not prove what you sent to the buyer or what they even returned. Photos or video are not proof, you could of swapped it out and taped it up. So eBay have to go by resetting the transaction. You get your item back, buyer gets his money back.
Unfortunately that will all come at your expense. Unless eBay rule in your favour for the appeal and then eBay will either refund the buyer or out of their pocket, send the sale price back to you minus fees.
After you get the item back, and that's if the buyer even send its back from China. You report a problem with the item before closing the return case. Call CS if you have to.
If the buyer dont send it back to you within the time frame, assume you can check a tracking number to see if it has even been posted. Call eBay CS and ask them to close the case. DO NOT escalate the case, this is done by robots and always against you most of the time. Talk to a human being.
Call eBay CS at 8am in the morning will increase you chances of getting either German or Ireland customer service instead of India or other aisa countries.
Others can advise more.
