The conventional wisdom "Buy the Seller" is one of the most important principles when buying pre-owned / secondary market.
Nowadays the super-clones are getting better and better. You have to be very careful.
In my opinion, if you do not buy direct from the AD, then the only way to 100% truly know if your watch is genuine is to examine the movement. Sadly, with Rolex, that means opening up the caseback and taking a detailed look at its intricate components, so you'd have to go to a legit, skilled watchmaker or a service center.
Of course, you can start by looking at the watch under loupe and making sure there are no glaring imperfections in the dial or dial apparatus, with great scrutiny at 10x or 20x magnification. A genuine Rolex will look essentially perfect under loupe. There are plenty of Youtube videos showing the distinction between genuine and fake. If you have a timegrapher, you can also check the accuracy and integrity of the movement to ensure that it meets technical standards of a genuine Rolex (very accurate, very little beat error, good amplitude, etc.) Also, a genuine warranty card will contain a hidden security feature under a black light (UV light) in the left upper quadrant. It's possible that the clone factories could counterfeit that feature too. It's a sad era for buyers who want genuine pieces but cant get them at an AD, and have to deal with possible fakes out in the wild.
When I bought my Omega AT and SMPc 300m, I got them from the secondary market and not straight from AD. I am able to see my 8800 caliber on the beautiful caseback, and I am 100% certain they are both legitimate. I tested them nevertheless with my timegrapher and under a loupe.
I'm also 100% certain my Grand Seiko is legit. Spring Drive is impossible to clone
