eBay never see the item you sent so always support the buyer, in line with the terms of the eBay Money Back Guarantee.
https://www.ebay.co.uk/help/policies/ebay-money-back-guarantee-policy/ebay-money-back-guarantee?id=4...
Has the buyer opened a case against you in the Resolution Centre?
All disputes must be settled through the Resolution Centre. Settling disputes by eBay messages are not enough as it can lead to complications later.
https://resolutioncentre.ebay.co.uk/
If they have not done so, ask your buyer to open a case and respond through that. No case? Then do nothing.
If/when the buyer opens a case, you have two choices; You can either reply within the case, Return for a full refund, issue a prepaid returns label, and refund in full when you receive the item back.
Or you can offer your buyer a partial refund and your buyer keeps the item.
This may be what your buyer is hoping for so it’s your call. In any event, you must resolve the dispute within 3 working days, which is when your buyer can escalate the case to eBay.
If you do not deal with the case within 3 working days the default eBay response is to refund the buyer in full at your expense and let the buyer keep the item.
Add the buyer to your Blocked Bidder List by eBay identity, and email address if you have it.
https://www.ebay.co.uk/help/selling/resolving-buyer-issues/blocking-bidders-buyers?id=4082&st=2&pos=
If you have not already done so, set your Buyer Requirements to block bidders with 2+ strikes in the last 12 months:
https://www.ebay.co.uk/bmgt/buyerrequirements
