THE ACFT DEPARTED ON A SALES DEMO FLT TO MONTEREY, CA WITH POTENTIAL BUYERS ABOARD. EN ROUTE, THE DESTINATION WAS CHANGED TO AVALON, CA. UNICOM ADVISED THE CREW OF THE WIND DIRECTION & SPEED ON DOWNWIND. WITNESSES REPORTED THE ACFT WAS SLIGHTLY HI ON FINAL, BUT SAID THE PLT CORRECTED THE ANGLE OF DESCENT. THE ACFT TOUCHED DOWN APRX 500' DOWN THE RWY, WHICH WAS CROWNED (SLOPED UPWARD FOR APRX THE 1ST 2000', THEN LEVELED OFF). THE CREW COULD NOT SEE THE END OF THE RWY AT TOUCHDOWN. THE THRUST REVERSERS WERE DEPLOYED APRX 1000' AFTER TOUCHDOWN. WITNESSES HEARD THE ENG SOUND INCREASE,THEN DECREASE, THEN INCREASE AGAIN AS THE ACFT WENT OFF THE RWY & OVER A 90' BLUFF. FIRE ERUPTED IMMEDIATELY. WITHIN 3 MIN, ARPT PSNL ARRIVED WITH FIREFIGHTING EQUIP, BUT THEY WERE NOT TRAINED AS FIREFIGHTERS & DID NOT TRY TO STOP THE FIREFOR FEAR OF AN EXPLOSION. NO EVIDENCE OF A MECHANICAL FAILURE/MALFUNCTION WAS FOUND. FOUND L FLAP EXTENDED, L SPOILER RETRACTED & LOCKED, TIRE MARKS ON RWY. ESTIMATED GROSS WT: 11,500 LBS. COMPUTED LANDING DIST OVER A 50' OBSTCL: 3100'.