This site is still in beta but it's pretty impressive and useful:
It's a big database of used bike prices. You enter the make, model and year of a bike by selecting from drop-down menus (if the bike's not in the database it won't show up in a menu) and you'll get details of the bike's original specs and price along with estimates for what a used model would go for in like new, excellent, good or fair condition. You can also search more generally for things like a "Trek Hybrid".
While the site doesn't have every bike and isn't really able to account for aftermarket bike upgrades or regional price differences (ie, used bikes prices are higher in places like Portland), it still is a handy starting point for figuring out if an asking price for a used bike is reasonable or just wishful thinking.
The site also offers a marketplace where registered users can buy and sell bikes. I've only browsed the site so I can't comment on the buying and selling experience.
I have no connection with www.bicyclebluebook.com but it looks like a quite well done. As more people use it, it's database should become even more complete. It's already quite impressive.
Kent "Mountain Turtle" Peterson
Issaquah, WA USA