flapbreaker-
As far as book value is concerned, the N.A.D.A. guidebook seems to be pretty accurate, but selling them and then making a deal on a new outboard and getting it installed are additional issues.
You can sell the outboards yourself, but still have to deal with the purchase of a new motor and it's installation unless you can do the install yourself.
Any dealer has to make a profit somewhere in any transaction. I got a great deal on my new Yamaha with installation and my dealer (Gunther's Outboard's of Bodega Bay/Sebastapool) sold my old motor and kicker for no commission as part of the deal. Give him or your local dealer(s) a call and see what they can come up with. It's a lot cleaner to do it all in one transaction and you have someone to hold responsible for the outcome, not a bunch of different deals. Just my four-tenths of a nickle. Joe.