I sold graphics cards & computer parts occasionally to make some side money. I wised up to it after a while, figured out what was going on.
There are powersellers on ebay who sell these same items and accrue returns/defective stuff over time. When they see a relatively low feedback seller like me selling the cards, they'd buy them under different accounts, file a dispute, then return their broken cards and keep my stock. And there was pretty much no way to argue anything with Paypal.
Last year I lost a grand because of a similar incident. Except I thought I was smart using UPS to ship with their "pack & ship promise", I figured even if something did happen I was covered because I had paid for insurance and had UPS pack the box. Well, no, they just denied the claim repeatedly. I just let it go eventually.
Ironically, I'm going down to Atlanta today to UPS corporate for a three day meeting. And less than six months prior I was so close to suing them.
If you're going to sell your watch (or any item) make sure you describe every last thing about it, even the serial numbers.