As above, private sales here is safest. Ebay is dangerous but chrono24 is more dangerous. Pretty much anywhere is dangerous if you don't know your target watch. Best to focus on a single model and study it intensively before buying. Asking for help on here is a double-edged sword; you get good advice on a watch you are looking at, but so does the rest of the world as anybody can look in, and believe me they do. So it can push up the price but that's still better than buying blind.