That's a beautifully simplistic example where all knowledge was known by all parties. The painting is there and there are not other strings attached. If, for example, there were actually 30,000 instead of just 30, maybe the price would be different. If someone else was willing to buy for twice that amount and you immediately flip to them, would the original owner feel bad about selling to you? Maybe. It gets tricky pretty quick when we factor in all the aspects of human emotions and subjectivity that go into valuing something.
The main issue I'm addressing in my post is where all the information isn't known and there's a major asymmetry of information one party is using against another. That's where it becomes less about a voluntary disagreement (and I wouldn't even frame it that way, more so a division of labor), and almost boarding on fraud where information is held back or kept private to obtain and unfair advantage.
