http://www.musicgoround.com/about-us.aspx
There are also some local guitar stores in some areas that specialize in used gear. Pittsburgh Guitars is one such store. They claim to "Buyeth, selleth, and tradeth".
http://www.pittsburghguitars.com/index.html
Ebay is a huge source of used guitars. It's also a great place to sell any that you buy and end up not liking. I bought my bass and amp on eBay, as well as tuners, stomp boxes, and miscellaneous other stuff. I know people who'll buy and re-sell guitars constantly on eBay. If you're a shredder who wants to try out a jazz box for a while to see if you like it, buy a used one on eBay. If you find out it's not your thing, re-sell it.
Check around for local internet forums. Many cities have "classified ads" websites that are only about the local scene.
Even guitar dealers who don't handle used guitars and won't take trade-ins will sometimes have an old-fashioned cork-board bulletin board where people can advertise used gear. The stores realize that if they help someone sell old gear they don't want anymore, the proceeds of the sale will end up being used to get new gear.
