"Guides" are just that, a "Guide"
Be it Parkers, Glasses, or Cap, they are there to guide you, like most things, these are not set in stone, nor do they give you a phone number of someone that will pay you that price.
Unfortunately, experience is the key to most things and know whether a car is either worth "Book", "into-book", or "behind book" and that's where experience of the Market comes to play.
From studying values, the guide I posted recently on here, that unfortunately has now been moved to the "Members Only" area, was fairly accurate, but like most things in life, what someone will pay for anything, is always a compromise between what the buyer will pay and what the seller will accept!
I haven't had a good laugh in ages though, so what did it say?