You have to pay extra for those seats, so the original guy who was sitting there went out of his way to book that seat. It is not clear how tall the guy was. I am 6’5” and almost always pay extra for the legroom. If my wife is with me, then I have to pay for one for her as well. If some guy wanted to buy my seat, it would depend on how bad his seat was, and how long the flight was and how bad I needed the money. It would have nothing to do with the fact that the guy looked like a black pimp.
I read there are health risks in flying long distances , as you described, sitting uncomfortable.
I always fly first class.
My comfort means a lot to me. I will NOT sell my comfort.