I've always been a PM guy but the posting of a successful purchase by a buyer on a recent sale saved me and another buyer from being scammed by the seller who took payment from all of us.
To respect the privacy of the buyer, I don't think their ID should be disclosed. Sellers should sell to whoever they want regardless of who PM first since it is for their protection and peace of mind to deal with someone they trust.
The seller should however update the thread as soon as payment has been confirmed. It is up to the buyer to post that he bought the watch if he wishes.