If 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.
This explains the appeal process:
https://www.ebay.co.uk/help/selling/managing-returns-refunds/appeal-outcome-case-seller?id=4369
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