The main question is, does it belong to the person selling it? Because they don't always, and that's one reason for bargain offers.
After that I suppose it's a question of "Is this fair?" - I mean you can make up O.Henry type stories where you'd have to be a real bastard to take advantage. This tattered and battered musician dies, his wife and kids in desperate straits, she's got no idea what his hallowed Gibson is worth and so on and so forth.
And if it was O Henry, the next twist of the story would be one it'd be wrong not to take advantage. Some unscrupulous bastard has bought the hallowed Gibson for a fraction of what he thinks its worth - but he's no idea of what it's worth either, so you give him what he thinks is a fair price, which is a fraction of its real value. I think it'd be immoral in such a case to enlighten him until after he'd parted company with it.
Situation ethics they call it, I think. Horses for courses.