I don't like taking Paypal payments for high value items because in the event of a dispute it's too easy for an unscrupulous buyer to claim they didn't get what they paid for and take their money back.
When sending the watch, I would make sure you take detailed videos of the watch whilst you package it up, weigh it and apply the postage label.
The other reason I don't like Paypal any more is that they now don't refund transaction fees if you refund the original payment (certainly this is the case in the UK now). I recently had a buyer ask to cancel an eBay transaction immediately after paying for it, which I accepted as I hadn't posted yet, but then found out that whilst I would get my eBay fees back, the Paypal fees were non-refundable. Luckily it was a low value item, but on a 15k watch, it's not insignificant!
Personally speaking, if I were you, I would try and cancel the sale.