If a case for item not as described is opened, it's all automatic, no point challenging it at that point with no human contact.
You need to accept it within 3 days, pay for that fully tracked return postage if you want your item back first, do not refund until it's back with you, you then have 48 hours to fully refund.
Stick within those timelines, and your seller performance is not affected.
Then, if you wish , you can appeal the decision for the refund. Link below explains the process.
I won't lie, it's not easy, as eBay do not see the item in question so not easy to judge who is telling the truth, and they do usually, believe the buyer.
https://www.ebay.co.uk/help/selling/managing-returns-refunds/appeal-outcome-case-seller?id=4369
@clairkno