All you simply have to say to a customer is "Go find one". If you are indeed selling a quality unit, and they aren't wasting your time and are actually interested in doing business, they'll be back. Regardless of book values.
A vehicle is only worth what someone will pay for it.
Look at some of the selling prices for old MB diesels, for instance. Do you think kbb or nada would quote a 123 300D with 70K miles at $15K? Nope.
