THE PERSON WHO SOLD THE ACFT TO THE PLT STATED HE HAD GIVEN THE PLT A TRAINING FLT THAT EVENING, BUT SINCE THERE WAS ONLY 1 SET OF CONTROLS, THE PLT (INVOLVED IN THE ACCIDENT) ONLY OBSERVED. SINCE DARKNESS WAS APCHG, THE PREVIOUS OWNER DID NOT WANT TO CONTINUE FLYING. HOWEVER, HE (THE PREVIOUS OWNER) SUGGESTED THAT THE PLT (NEW OWNER) COULD TAXI THE ACFT, WHICH WAS A 2 PLACE ULTRALIGHT. WHILE THE PLT WAS TAXIING, WITTNESSES SAW THE ACFT BECOME AIRBORNE. ONE WITNESS SAID THE ACFT CLIMBED TO 150TO 200 FT, ENTERED A SHALLOW RIGHT TURN, THEN ENTERED A LEFT TURN WHICH BECAME PROGRESIVELY STEEPER UNTIL THE ACFT WENT INTO A STEEP DIVE & CRASHED. TWO OTHERS SAID THE ACFT ENTERED A STEEP CLIMB AFTER LIFT-OFF, THEN ENTERED A LEFT TURN. ONE SAID IT MADE TWO 360 DEG CLIMBING LEFT TURNS, FOLLOWED BY THREE 360 DEG DESCENDING LEFT TURNS, BEFORE IT CRASHED. ALL SAID THE ENG REMAINED AT HIGH POWER. THERE WAS EVIDENCE THE PLT USED A 10 LB WT ON THE RIGHT WING FOR BALLAST. REPORTEDLY, THE 210 LB PLT NEEDED ABOUT 11 LBS FOR LATTERAL BALANCE.