Well, there's the marketplace subforum here which you can use. There's also Omega Forums, Watchuseek and Rolex Forums (the latter you have to pay to advertise on). If you can stomach the fees, then eBay is possibly the best place these days, with the authentication programme adding confidence (you ship the watch to an independent checker and they forward on to the buyer). There's also Chrono24.com which gives you a wide audience, and they offer an escrow service. You might struggle to sell on forums without any feedback, though, so maybe eBay or Chrono24 is best for you?