The non-certificated pilot/owner of the single engine airplane was conducting a local test flight with a prospective buyer, who held a private pilot certificate. After exiting the airport traffic area and climbing to an altitude of 1,800 feet, the pilot permitted the prospective buyer to manipulate the flight controls. The pilot then reassumed control of the airplane for the return flight to the departure airport. According to the prospective buyer, while on final approach to land on runway 5, he noticed that the airplane was yawed to the left and applied right rudder, but the pilot told him to stay off the controls, which he did for the remainder of the flight. After landing, the airplane veered to the left and departed the left side of the runway; where it struck an embankment, and flipped over. The airplane sustained substantial damage to the wing struts, rudder, aft lower fuselage, firewall, and nose gear assembly. Both occupants stated there were no pre-impact anomalies with the airplane.