I can't say with authority what is the secondary market for a 5970, other than its at or above MSRP. My guess is that 5970's, particularly WG, sell for at least 10% above MSRP on the secondary market, and maybe substantially more.
An individual has a much tougher time selling a $100k+ watch than will a dealer, AD or gray. Individual on these collector watch sites does not have the market penetration that a dealer has, and there is the issue of buying a $100k+ item from an unknown individual in a marketplace rumored to be rife with fakes, overstatement, and fraud (I am not speaking of watch sites imparticular, but the whole "fake Rolex" issue, etc.). People able to buy this kind of watch don't want or need to take the risk, real or imagined. Additionally, almost all individuals want a no-recourse transaction (for fear of what might have happened to a watch while out of their hands) where a dealer has the expertise and the business motive to be more generous. Buyers wouldn't believe they would get there money back from an individual anyway.
The seller of whom you write sold to the gray market at wholesale. The piece will be resold for a signifant profit over what your seller got, and there is a reason for the gray dealer to get that markup. If you think the dealer bought it as part of some conspiracy with other gray market dealers to artificially inflate the price, you live in a different world that me. Rather, I would suggest he bought it at a price at which even in a bad market he will make some decent money.
Its really not that much different (but maybe worse) than trying to sell a $100k car across the country, based on a few pictures and without the benefit of Carfax or other service that can speak to the history of the vehicle. Dealer will always do MUCH better.