You have messages hopefully through eBay's messaging system that they ' fiddled' with the item so the blame here lies with the buyer for breaking off this delicate piece on the item. An unfortunate choice of words but it does imply the fault lies with them.
In my humble opinion this is not a case for item not as described, if that case was opened, hopefully your messages would help your side of the matter.
They haven't asked for any refund, so it's up to you to either just explain that was always a piece to handle with care , or to keep them happy, you can agree to offer partial refund, though you are under no obligation of course, to do this.