You as the seller, are responsible for getting the item to the buyer in perfect condition. Your buyer can open an ebay case for "Not as Described" and you'll have the choice of either refunding without return, or providing a tracked label for your buyer to return for refund.
You in turn make a claim from Royal Mail, but you will probably need to get the parcel back from your buyer to start a claim as you will need all the packing, and any correspondence from RM.
Was the part placed in a clear plastic bag for instance, with an explanation on it? That's what RM do when there is obvious damage to the item.
