Ebay - Buyer claiming "return request" for SEALED ITEM

Hi all,

I know this has been asked many times....!

I recently sold an BRAND NEW SEALED ITEM.
I mentioned in listing that item can only be returned if the SEAL is intact (I.E. Not opened)
Anyway the seller after 14 days has opened a return request saying "item defective" no other explanation given. They have used item!!

What are my choices/options..

Cheers & Stay Safe