I think we've already covered this to some extent on the other thread. Ill tell you my experience though.
I have only bought once through Chrono24. It was a new Breitling for my wife. Seller advertised the watch as having full manufacturer warranty but the card wasn't stamped. I returned for a refund.
I have sold one watch as a private seller. The buyer had to confirm receipt of the watch before funds were released to me via their escrow service. It was a smooth transaction.
I also found my Carrera reissue on chrono24. But in that instance I found the seller's details (a dealer in Italy), looked them up and purchased direct at a small discount.