This is a discussion on Where to buy USED watches? within the Watches forums, part of the Everything But Cigars category; What are the better places to buy a pre-owned watch? I really like the looks of some of the older ...
There are good sellers on ebay, but you have to be careful. On an expensive purchase, use an escrow service.
I've had good luck with smaller jeweler that aren't authorized dealers for the brand I'm looking at. They don't have the selection, but they take a lot in on trade. If they have the model you want, you can usually get it cheaper there than a used one from an authorized dealer.
Ebay and watchuseek.com (watch forum). Try to use creative search terms and you can find watches no one else can because of how it was listed. If you want it from an authorized place, any jewelry store will have them. Really depends on what you are looking for whether it be an old Seiko for $50 or an old $5k Rolex.
More and more brands are opening up either their own e-commerce websites or are authorizing their dealers to sell watches online. These are often the safest places to buy watches on the internet, and the watches will come with factory warranties. Ebay is always the best marketplace for buying used watches. A lot of online e-commerce sites and dealers sell rolex watches at really affordable prices.
I've gotten a few cheaper watches on ebay, but you have to be careful.
I've had good success as a member over at watchuseek.com. They are some good guys. I've picked us several used watches from the classifieds on the site.