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.
The Blue Book value of a car is the dealer or "wholesale" value of a car. It's unlikely that a buyer would get a better deal than that.
"A fraction of Blue Book value" means "cheaper than the best possible deal," i.e. one that's "too good to be true."
The only way that someone could "reasonably" offer such a deal is if the goods were stolen or counterfeit.