I don't think it works as any valuation on a car one would wish to trade with a Dealer (for one of his cars) really depends on the Dealer and his Red Book valuation. I recently called in at a yard with a number of quite attractive vehicles in his yard. I had my Captiva parked in front of a store about fifteen metres away.
I said, after some discussions, why not have a look at my car - it's all there for you to see and evaluate, and give me a price on your $17K. Kia Sorento. Just not a 100% sure which car - for the purpose of this post, it is of no real consequence. The Captiva was, or is, in really nice condition, straight, nice paint, nice everything, and it was indeed ready for a fair valuation. On returning to him to ascertain his thoughts on my car, he was happy to tell me that, according to the Red Book, the car would be worth between $700.00 to $2000.00, depending on......? He was not really interested in even looking at the car, nor its condition. He knew it was a five seater (could or should have been seven) which would affect its trade in valuation and he was only interested in what the Red Book told him.
In my view, always best to advertise privately, preferably to "passing" motorists on main roads. Regards Styria.......unless, of course, you have something really super on your hands.