If it's the one you sold on 13th March then depending upon the delivery time the buyer probably has just a few days (30 days from date of delivery) in which to open an item not as described case under eBay's money back guarantee.

I'm afraid they're not required to give you prior notice or give you any further information. eBay will probably require a photo but cases are decided without eBay even seeing the item, and usually in the buyer's favour. As a private seller you can't make any reduction based on the condition in which it is returned.

You can theoretically appeal the case on return although the most likely outcome is that eBay will still support the buyer, and place a defect on your account. If you believe the buyer has abused eBay's money back guarantee you can report this to eBay.

The procedure is very much weighted in favour of the buyer.