How do I buy a used bike?
How do I evaluate the value of a used bike?

Well, the simple answer is a bike is worth as much as someone is willing to pay for it. 

As guidelines, a few websites may be useful:


Craigslist (See what similar bikes are going for in your area)
EBay (Online retailer)
Bicycle Blue Book (Gives values for certain bikes; can be useful as a rough guideline)
Bikepedia (Gives values for certain bikes when new, along with their original components)
Velobase (Database of vintage cycling and marketplace)