Offering 50% on a situation like this is a joke IMO. You sent the 'watch' in a shitty padded envelope. And this is after multiple previous sales went bad because you sent the watch in a shitty padded envelope. Any normal, rational person would have learnt from previous mistakes and adjusted how they shipped watches in future transactions, but no, you chose to ship in a SHITTY PADDED ENVELOPE.
Are you fucking dumb?
Have you learnt nothing from past transactions that caused you a fucking headache?
If you went out, leaving your front door open and returned home to find you'd been burgled; sure as hell is hot, you'd close and lock the front door the next time you went out. But if it had happened to you twice, you'd Fort Knox the place with added security, CCTV and alarm systems to be extra vigilant that it never happened again. I don't get the mentality of shipping in a padded envelope.
IF (in capital letters to emphasize the point) - IF we were to believe that you actually shipped the watch and that it was stolen in transit; you have directly attributed to that issue by sending it is a SHITTY PADDED ENVELOPE where it was obvious to any postal worker handling the package, that the content was a watch. Therefore, you are at fault here. Personally, I do not believe you sent the watch. I have seen no evidence at all that you shipped a watch. If you want your name cleared, EVEN IF the buyer accepts any partial refund; I would expect that you post a copy of the full postage receipt (redacting the buyers name and address) which would show the weight of your parcel. That would go some way to demonstrating the potential that the package you shipped had contained a watch. Personally, I would still expect a full refund due to the shitty method of shipping, especially after this has caused you problems in the past.
I would also have expected a genuine seller to have responded in such a way as to ask the buyer for all evidence possible of the condition of the package received, and the contents, so that I could have started an investigation with the postal service. But that has not happened here, which makes me wonder why? Again a rational person and honest seller would do so.