Say, Person A owns a rare postage stamp which is worth 2000€ according to the stamp collector community, but A doesn't know the worth.
Person B sees that A has a rare stamp and offers to buy it for a fraction of its worth. B offers A 100€ for the stamp and A agrees happily.
Later, A finds out the true value of the stamp and wants it back. A claims that B scammed them.
Is B guilty of fraud, simply for not telling A how much the stamp is worth?