How do I buy a used bike? Since we have a lot of questions asking about buying a used bike, I thought we could do a community wiki on how to buy used bikes as a be-all, end-all.

There are a few main questions:

1) How do I check if a used bike is safe to ride ?

2) How do I assess the value of a used bike?

Feel free to add others as answers, one question per answer.
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)