On this page : http://boston.craigslist.org/i/autos the fourth point of "How to recognize a vehicle scam attempt on CL"
What does "fraction of blue book value" mean?
The Kelley Blue Book is a commonly used resource to value new and used cars. Fraction of the blue book value presumably refers to a price that is a fractional multiple of the blue book value.
As it relates to scamming on craiglist, I would imagine that many scam artists try to draw in potential victims by claiming they're selling a car at a fraction of the blue book value.