If your buyer opens an eBay item not as described return, they'll be required to send it back to you for a full refund of their original payment, and you'll need to provide them with a pre-paid returns label (if you don't provide a label, the buyer can escalate the dispute and get one from eBay, who will re-charge you for it.
You will get a defect against your selling account if you don’t accept the return and the buyer escalates the dispute to ask eBay to step in, which they can do after 3 business days, so best to just accept the return.
eBay can’t see what state the item was in when it arrived with the buyer, so have a resolution process in place which tries to put both parties as close as possible to their start position. Unfortunately, sellers do lose out on postage both ways.
If you feel that your buyer has misused eBay’s Money Back Guarantee you can use the Report Buyer link next to the item in your My eBay.
If you receive your item back and it’s not the one you sent, or it’s damaged or has missing parts you can appeal to eBay within 30 days.
Your options for responding to a return request are on this eBay page:
https://www.ebay.co.uk/help/selling/managing-returns-refunds/handle-return-request-seller?id=4115&st...
I would add that buyer to your Blocked Buyer list though, you don't want to deal with them again.