I have made 9 watch purchases online, and in each case I sent full funds in advance via either TransferWise or PP F&F. I knew it was risky, and that in each case I would potentially lose all of the funds. However, each seller had enough posts on OF for me to feel confident they were not a fly-by-night seller/scammer and that they valued their reputation. My total spend was around $13k USD (so lower valued watches than what you're considering), but for me they all worked out.
FWIW, I have recently listed a few watches for sale and I would be okay with submitting evidence (licence, passport, etc) to a buyer if needed.
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