I suggest you read the user agreement.  You will see that you agreed that ebay has the right to decide disputes.

You should also read the money back guarantee guidance.

You could try and get the item back from the buyer but not through ebay.

You are not the only one to make this mistake.  See here, it was a collection item but that makes no difference. 

https://community.ebay.co.uk/t5/Seller-Central/Ebay-wrongly-issued-refund-breaking-own-rules/m-p/801...