Have they actually opened a case for item not as described? If not , you can send them a polite message that taking apart an item is against eBay's rules to receive a full refund, the item has to be returned in exactly the same condition as sent out, the issue could have been caused then, with disassembling the item, and videos were previously provided to show the item in full working order.
This may appease them, if not and they do open a case, for item not as described is opened, you will have to accept the case within 3 days. Cases are all automated no point challenging it at that point.
If you want the item back, pay for a fully tracked return postage, and only refund through those case details, when the item is back with you. You will have 48 hours to do so.
Stick within those timelines, and your seller performance will not be affected.
If you think the refund was unjustly granted, and the buyer has abused eBay's 30 day MBG, Money Back Guarantee policy, you can appeal the decision. I won't lie, it's not always easy, and does favour the buyer in most cases, but you can direct CS to the messages the buyer sent informing you they took the item apart, and as a video showed it working previously taking it apart could have caused the issue.
This explains the appeal process:
https://www.ebay.co.uk/help/selling/managing-returns-refunds/appeal-outcome-case-seller?id=4369
@nickfocus170st
