If an individual buys an auto from someone who still owes on the auto and finances thru other means, do they tell the purchaser the amount of the payoff and what profits the seller may make?

What does the existing loan balance have to do with profit? I buy a car for $40,000. I sell the car for $20,000 while I still owe $10,000. Explain to be where my profit might be? Regardless - how much the seller still owes on the car is really none of the buyer's business.