The Twilight Zone - The Whole Truth

Harvey Hunnicutt is the stereotypical used car salesman: a fast talker who, to put it politely, is prone to stretching the truth about the cars he sells. He buys a used car from an old gentleman paying him far less that it's worth. After the deal, the old man tells him the car is haunted. Soon, Harvey finds that he can only tell the truth. Not only to customers but even to his wife as well. When he tries to sell the man's car he finds the perfect customer