One simple trick for making a low-ball offer is to preface it with "...this is not what I think your plane is worth, but all I can afford to pay is $xx,xxx".

Takes some of the sting out, deflects the blame, and helps keep the negotiations civil.

-Kirk Wennerstrom
President, Cessna 150-152 Fly-In Foundation
1976 Cessna Cardinal RG N7556V
Hangar D1, Bridgeport, CT KBDR