I also reccomend Craigslist..but as Cody21 said, be careful.  As a buyer, I got scammed once, but otherwise have bought and sold numerous times very happily.  Stay in public, and don't exchange cash or merchandise until you are very comfortable with the situation.  I bought my Droid X of of craigslist for 80 bucks but I've seen them in my area for anywehere between 100-150.