Here's some of my personal advice when buying from classifieds and forums:
a) ask if seller has references.
b) Check if he has an ebay account. If he has a high feedback ebay account, send him a message via ebay. If he replies via ebay its a good way to identify the person (even if you are not buying via ebay).
c) Ask him to set the watch to a specific time and date. Get him to email you the photos of the custom setting YOU chose.
d) if he has the original receipt, ask for a photo
e) When you get his email, put the IP address headers into an IPLookup. If the emails do originate from another area which he claims to be in, its not good (this may not work if the seller uses free email accounts like gmail, yahoo, hotmail etc)
f) check his post count. Then go to his profile and look at his activity. If he has thousands of posts and they look legit, then its a good thing. If he has low posts and most are just random senseless replies & threads, then its not so good.
g) when paying, ask for a paypal invoice that clearly states what you are getting. Get him to send a payment request first. Get seller to use words like Genuine, authentic etc. Fund from your credit card.
Then its all up to gut feeling.