Thanks for your great advice, Jeff! This set of questions is really and imprecindable starting point.
The concept of "buying the seller" certainly mitigates the risks of entering a negotiation, but there few guarantees. Two different scenarios can occur: 1) The seller himself is honestly unaware that there is a problem with the watch; 2) The seller is an expert crook.
In first case, getting additional expert opinions or adopting a more conservative approach (like not closing the deal unless you get definitive proof of the piece's authenticity) may help.
The problem is when the second case happens. I recently read an article about a case of falsification of special wines. The defendant in this case is one of the most repected wine collectors in USA, famous for investing more than a million dollars each year in rare bottles of wine. So, these "experts" are sometimes very keen in building their reputation and than, taking advantage of that.
It's a wild world out there.

Have you all a nice weekend - and take care... :)