After refueling his airplane at an intermediate stop and while waiting to start the engine after an unsuccessful attempt, the pilot was approached by an airport tenant who offered some alternate engine-start advice. When subsequent attempts to start the engine failed, the pilot decided to attempt an engine start by manually rotating the propeller while the tenant adjusted the engine controls from outside the airplane. The engine subsequently started at a high rpm. The airplane departed from the chocks and struck an unoccupied parked airplane, which resulted in substantial damage to the left wing and fuselage. The pilot was able to shut the engine down and observed that the throttle was in the “halfway position.” The pilot did not report any mechanical failures or malfunctions with the airplane that would have precluded normal operation.