Hi Dee,
I can't guarantee that any method is fool-proof, but here are some of my personal thoughts:
1 - usually in a public place, but not out in the open (in case they run off!) - eg. Starbucks, at a not so busy corner, etc.
2 - let them hold briefly, inspect. if possible, take their other watch as collateral (if its worth as much haha). I usually dont let them try to wipe the watch, or wear it on and walk around. Just seated, calm inspection.
3 - Cash if below RM5k I guess is OK. But anything above, online transfer. Standby a laptop. Make sure you determine with the buyer in advance how he/she intends to pay you. If online, remind them to unblock their limits - sufficient to enable the full transfer. Never accept partial transfers and promises to do a subsequent one after they take the watch! If before they take the watch - then OK!
4 - Be observant of your surroundings. If the person is a crook, chances are he may have an accomplice nearby. If watch goes "missing" suddenly, the accomplice could have taken off with it without you knowing. Before purchase, get to know the buyer a little more too.
Oh, and if your gut-feel tells you something's amiss, then it highly likely is. Personally, I would rather wait for a more secure deal. But again, just my opinion.
Just my 2 cents, all the best in your sale! Cheers