It appears you want to know how to SELL a watch safely. If you know the buyer then you can relax some rules. If you don't know the BUYER, then..
1. Choose an enclosed location like Starbucks in a mall so buyer cannot just bring a few friends to come and snatch the item away and run.
2. Allow inspection on the spot but do not let the watch leave your sight. No opening of caseback etc or other actions that could damage the watch.
3. Transaction by Instant Transfers only. Ask buyer to use multiple accounts to transfer. Or spouse account. I think CIMB account can transfer up to RM250k daily.
4. No GIRO. No CASH.
5. Make buyer aware absolutely no refunds, returns, remorse once watch leaves your sight. He can still go make police report later and said you con him/her so think of ways to make sure he has no legal means to con you.
But to be honest, if I was a buyer and the seller made such rules, it would be difficult to proceed because seller could also be a con-man. Of course you could ask to have the watch inspected at a reputable watch shop but unless you know the owner, how can you be certain they are not working together to con you? And do you personally know any watch shop owners?
This is why most people prefer to just sell to the shop and avoid the hassle.
If buying, buy the seller and likewise if selling, know the buyer.
BTW, what watch are you planning to sell that is more than 50k?