There is a difference between "unjust" and "not justified". One is saying the price was wrong, the other is saying it was just arbitrary/irrational. As far as I know, being an idiot is not ethically wrong, just unfortunate.
But yes, we can indeed deduce that all art sales may very well be unethical insofar as the seller knows and makes use of the fact that they're dealing with idiots, because they depend on selling something inherently valueless at an arbitrary price. What actually costs money is the labor and materials, so the price should be relative and proportional to the time and resources spent to count as a rational exchange. Of course the normal rules of supply and demand apply - an expert's work is worth more than an amateur because it's in shorter supply.
Sotherby's art auctions are basically people who are trying to play hot potato with art. They jack up the price from buyer to buyer until someone gets left with a "priceless" piece of art they can no longer sell. They're the loser of the game.