THE ACFT DEPARTED THE PRIVATELY OWNED DIRT STRIP ON A SALES DEMONSTRATION FLT. THE PIC OCCUPIED THE REAR SEAT AND THE PROSPECTIVE BUYER OCCUPIED THE FRONT SEAT. SHORTLY AFTER DEPARTING THE ENG BEGAN TO RUN ROUGH. THE ACFT ENTERED INTO A COURSE REVERSAL TURN. DURING THE TURN THE ACFT STALLED. THE PLT WAS ABLE TO RECOVER TO A WINGS LEVEL ATTITUDE PRIOR TO THE INITIAL GROUND CONTACT. AFTER GROUND CONTACT THE ACFT BOUNCED AND STRUCK THE GROUND IN A LEFT WING/NOSE DOWN ATTITUDE. THE LEFT WING FUEL TANK LINE RUPTURED AND A POST IMPACT FIRE BEGAN. THE ACFT ROTATED TO THE LEFT AND CAME TO REST RIGHT SIDE UP. THE OCCUPANTS WERE UNABLE TO EXIT THE BURNING ACFT. THE ACFT HAD NOT BEEN FLOWN WITHIN THE PRECEDING FOUR MONTHS. GROUND WITNESSES COULD NOT STATE IF EITHER PLT HAD PERFORMED A PREFLIGHT INSPECTION AND IF THE FUEL TANKS WERE DRAINED PRIOR TO FLT. FUEL SAMPLES TAKEN FROM THE FUEL SUPPLY TANK WERE FREE OF ANY CONTAMINATION. THE ENG EXAMINATION DISCLOSED NO EVIDENCE OF ANY PRE-IMPACT MALFUNCTION/FAILURE.