The certified flight instructor (CFI) had previous experience flying the experimental, homebuilt, model of airplane. He offered to provide the prospective airplane purchaser with a familiarization flight that included stalls and landings. The CFI demonstrated a couple of stalls to the student, who held a private pilot certificate and a tail wheel endorsement. Thereafter, landings were practiced. The CFI reported that he was monitoring the pilot's performance while the pilot was handling the controls. The CFI stated that the pilot's first landing was not satisfactorily performed, and the pilot almost lost control of the conventional gear airplane during rollout. The pilot's second approach and 3-point touchdown appeared satisfactory. However, after the airplane rolled several hundred feet, the private pilot suddenly and abruptly applied left and right rudder pressure while moving the flight controls erratically. The CFI joined the pilot on the flight controls in an attempt at maintaining control. The CFI could not overcome the erratic inputs of the private pilot, and directional control was lost. The airplane's divergent rollout course was not corrected and the airplane exited off the left side of the runway and ground looped. The airplane came to rest with collapsed landing gear and a bent wing.