At 18:00 CST, a Cessna 172L was damaged when it struck a sea wall and hangar during engine start at Lakefront Airport. The pilot, who hand propped the engine, was not injured. The pilot said the starter would not engage properly, so he turned off the ignition, put the keys in his pocket, pulled the throttle to the off position, set the parking brake and got out of the airplane. He chocked the airplane and then slowly rotated the propeller until he felt one piston move to top center. He then got back in the airplane and attempted another start, but the starter still did not engage. Once again he turned off the ignition, put the keys in his pocket, pulled the throttle to the off position and got out of the airplane. He was not sure if he pulled the mixture to the off position, but he thought he did. As he again slowly moved the propeller, the engine started. The airplane jumped the chocks and traveled about 150 to 200 feet until it collided with the sea wall and a hangar.