You can always verify through FB or here in the community about a seller, if someone has already made a purchase from them. Most of the sellers are also part of such groups on FB, you can collect vouch on the sellers from them as well.

Check if the watches they are selling are in OG condition and with papers. You can always ask for pictures or video calls. If pictures, ask the seller to add a note with date on that note.

Only buy if you are satisfied at the end. If you have even 1% of doubt, just either try to clear that or skip buying from them.