The pilot of a retractable landing gear airplane reported that during dusk conditions, five nautical miles from the destination airport, he extended the landing gear. He further reported, that at touchdown the airplane started to shake. He allowed the airplane to slow down then applied brakes; the left main landing gear folded back into the fuselage, the airplane skidded down the runway, veered off the runway to the left, and impacted a runway exit sign. The airplane sustained substantial damage to the empennage. In a statement provided to the National Transportation Safety Board by the airplane mechanic that assisted in the recovery of the accident airplane, he reported that the accident airplane was lifted off of the surface of the runway, the left main landing gear was put into the down position and the up-lock hook which secures the landing gear in the down and locked position, "went into position with no problem." He further reported that he placed the airplane on jacks, cycled the landing gear through several cycles and the landing gear locked into position during each cycle. In the section of the National Transportation Safety Board Pilot/Operator Accident/Incident Report form titled Mechanical Malfunction/Failure, the pilot reported that during the postaccident repair of the airplane, the selector-main gear actuator (part No. 12810044) gear tooth was revealed to have been broken. According to the manufacture, the landing gear could have been extended with the broken gear actuator tooth, however once the landing gear had been extend, locked, and weight apply, the only way for the landing gear to unseat from the landing gear pocket is through a positive action by the pilot.