The only reason you are upset is because you are a good conscientious seller. Anyone else would ' take it on the chin' as they say, accept the return, add any postage loss to another item, so they're not out of pocket, and block the nuisance buyer!
eBay do not see the item, and it is hard to judge from photos exact colours I had to accept a return for a skirt I described as mauve, buyer insisted it was brown ??? It was refunded, buyer blocked , and I carried on.
Don't let it upset you, you cannot argue a case in progress, it's all automated, but can challenge a refund after if eBay grants it. In your case as said , eBay do not see the item , and doubt you'd win an appeal.
Truly not worth the stress and worry, just block this buyer adding their ID here:
https://www.ebay.co.uk/bmgt/BuyerBlock?
@sulusail