A guy I know who is mentally ill and wanted to get $30 and use it on a punch card (gambling). He sold me his BH Landstar for $30. Should I have told him that it was worth more? After all, he would have just sold it to someone else for the same amount, or less..
In this case this is not a test of morals or religion or kindheartedness, it is a philosophical delimma; Should I take back the coat that was worth more than the store asked for it, or should I have sent them a check for what difference I thought they should have asked for it? The man wasn't stupid, he was just mentally ill, and that illness had nothing to do with his ability to make a good financial decision.
On the other hand, what if I worked for $10 per hour when someone should have been paying me $30 per hour? How about the other way? Should I tell them that I am being paid too much at $30 per hour instead of $10?
We owe much more to those we know and love or who are weak or incapable, than to those who or strangers or casual aquaintenences. We have a stronger sense of duty and responsibility for them. Family and friends and helpless people usually get more from us than do those who we know little about. If it was my daughter, a friend, or someone else familiar or quite obviously content with their asking price and they appeared to be able to know the value of it I would not let it worry me, but I would offer what it was actually worth. If it was a little old lady and she didn't know that she was selling something for too little $$, I would tell her so and offer her more. If she refused my offer and said she already knew the value but wanted to sell it anyway I would not question her motives, I would simply go with her wishes instead. I have sold things too cheaply to people or given it to them, simply because I wanted to help them, or to be kind, and not to make a profit. I wouldn't challenge her motives, it might even offend her.