The pilot reported that while flying on the downwind leg of the traffic pattern in a single seat, tailwheel-equipped ultralight type airplane, he observed a Cessna airplane takeoff from the runway. When the Cessna was clear of the runway the pilot conducted an uneventful wheel landing. During the landing roll, as the airplane approached the area where the Cessna lifted off, the airplane was “picked up” from the runway. The pilot applied full power but the airplane was “slammed down hard,” substantially damaging the wings and fuselage. The pilot reported no mechanical failures or malfunctions with the airframe or engine prior to the accident.