The pilot/owner and a commercial-rated pilot departed in a tandem, two-seat, tailwheel-equipped airplane on a local flight. On the return leg to the airport the commercial pilot was flying the airplane. The commercial pilot, who was seated in the rear seat, did his before-landing checks to set up for the landing; however, during the approach to the runway the engine lost power. Unable to regain engine power, they elected to conduct a forced landing in a vacant lot. During the forced landing the airplane sustained damage to the left wing and fuselage. After the accident the pilot/owner and commercial pilot discovered that the engine’s fuel shut-off valve was in the (pulled) off position. The commercial pilot reported that he flew a Cub Special (PA-11), but never the accident airplane (J3), and that the fuel shut-off valve in the J3 was in the same place as the carburetor heat in the PA-11. The commercial pilot added that the shut-off valve was not color coded or marked, and that he pulled the fuel shut-off knob mistaking it for the carburetor heat.