I personally have no problem buying used, and all the watches I've ever bought except for three have been used. I wouldn't worry too much about warranty as I've never had a single watch go bad on me. If you look at the used market, $500 can get you a lot of very nice watches. My favourite in that price range would be the Seiko Sumo.
You can check the watchuseek.com for sale forum which has the most postings, or if you know specifically what you want check elitedealseeker.com which searches all of the popular watch forums.
Bookmarks