During the preflight inspection the pilot noticed a small amount of hydraulic oil pooled by the trim wheel. In order to trouble shoot the problem maintenance personnel removed the left front pilot's seat and then reinstalled it when the problem was resolved. After adding full power and during the takeoff roll the pilot's seat abruptly slid to the full back position, resulting in the airplane veering sharply to the left as the pilot's feet left the rudder pedals. In order to avoid taxi signs on the left side of the runway the pilot rotated, but then he heard something strike the airplane. Airborne, the pilot managed to get the seat forward, climb to pattern altitude, and land uneventfully. After securing the airplane the pilot noticed that the left front seat had not been reinstalled in the seat rail, but was resting on top of it. Damage to the airplane consisted of a propeller strike, a hole in the undercarriage, and damage to both horizontal stabilizers.