What's stopping the buyer from saying he hasn't received private documentation plate and e Bay not paying the money .With a dispute which could take an eternity to sort out as outside sources would have to be tried to be brought into dispute to identify they've actually had them.
Provided you sent it by a tracked delivery service and uploaded the tracking number when marking it as dispatched eBay will accept this a proof that the item was delivered. (Assuming that it was delivered, of course!)
