Here's another situation to think about. Someone is selling a very rare and expensive car, say a 71 hemi cuda convertible and has no idea what it is worth and someone who knows what its worth buys it for say $5000 and turns around and sells it for over a million. Should the person who bought the car and flipped not done so and explained to the seller what the car was really worth? 
For me, that situation is very context sensitive. If there's a 71 Hemi Cuda and the owner is-
-a widow whose husband bought the car and parked it years later, and she's on welfare and about to lose her house, I'd feel compelled to tell her what she had.
-a guy in the Hamptons who has his butler show me the car because he's busy entertaining Halle Berry on his yacht, I write a check and take the car home.
Not that I think the damage to the rich guy is, overall, any less. I've known several very wealthy people in my life; my father-in-law was a billionaire (sadly, none of this rubbed off on me). And while I would think, from my standpoint, that he had "enough" money, what I learned about him, and many of the other wealthy people I've known, is that each dollar is like a child, and the loss of a single dollar was like the death of a child. I was privy to a deal in which a guy lost $100,000 (out of a personal fortune of roughly 25 million), and he was miserable, couldn't get over it.
But, I wouldn't be affecting the quality of the rich guy's life. He'd be doing that himself, by not being able to let go. I would be affecting the quality of the widow's life, and I just couldn't bring myself to do it.
