Research is key; there's no single place to buy preowned timepieces.
I usually thoroughly browse eBay, Chrono24, and classifieds on watchuseek (and here).
When I see a nice watch on ebay deemed overpriced, I just add it to the watchlist.
Even if takes months, and it is not unsold, the seller will eventually come to reason and accept a best offer.
Not always the case, but it sometimes work.