"Fair value" is a very difficult concept to pin down. Most potential buyers and sellers don't agree on it which is why, very few websites change hands.
With that said, I just bought one (and have a blog posting on
acquiring websites) and have bid on another. I've found them thru personal contacts. I only buy established sites in my industry (kids education).
In terms of resources, I looked at
www.sedo.com but preferred:
www.sitepoint.com . In fact, they had a great free guide to
Buying Websites.