If your buying something very valuable but the seller doesn’t know do you need to inform him first?
I’m also thinking about the sale of the bechora from Eisav to Yackov. Eisav may have known the value and was still mivazeh it
A buyer that realizes that the item he is buying is very expensive and the seller is selling it for cheaper than 86% of the items value has to inform him, that the item is worth more than he is charging. However, if the seller is a non-Jew, and he isn’t fooling him into selling it for cheap, rather he fooled himself, then it is permitted.
Regarding Yackov and Eisav, The Ran (Drashos -2) asks your question. It seems that Yackov bought the bechora, which is priceless, for a pot of beans, so why isn’t that ona’ah? Additionally, he asks that the idea of the bechora was also a status symbol, because it is the one who is more prominent in the father’s eyes, and this is not something that is priceable? He answers that the day of this story was right after the funeral of Avraham aveinu. Esau didn’t really care that his grandfather died, because he didn’t go the funeral, (as the verse says that he just came in from the field), additionally, he wasn’t concerned with the fact that his father Yitzchok was sitting shiva and these beans were for him to eat. All he was interested in was to eat the beans. This enraged Yackov, that he was so careless to the honor of Avrohom and his father Yitzchok. Therefore Yackov told Eisav that he will give him the beans, but on the condition that he sell the bechora. The honor of bechora is because the honor and prominence of the father essentially rests upon the eldest son. Being that Eisav doesn’t have such a regard and relationship with Yitzchok, and Yackov is the one fostering that relationship, therefore Yackov demanded that the status of their relationship with their father remain the same from now on. Yackov is in the house all the time tending to his father, and Eisav is in the field, leading a different type of life, therefore Yackov demanded that he be the one to have the special relationship with Yitzchok. This special relationship is interpreted to selling him the bechora. To this Eisav replied, “I am going to die anyways”, meaning that the righteous will go in the ways of their righteous parents, but I have chosen a different type of lifestyle for myself. Therefore, what will I have from Yitzchok’s name, since he sits at home, leads a different type of life with different values, his honor has nothing to do with me, as I have different values. Therefore I don’t need the bechora.
According to the Ran, Eisav knew the value of the bechora, but scoffed at it, as the torah writes “vayivez Eisav es habichora” that Eisav degraded the bechora. Therefore it was not an issue of onaah, because he knew about it well.
Pischei Choshen (Mechira) 11-1, ftnt. 2 brings the Rema CH:M 348-2 that we may not fool a gentile, however if the gentile makes a mistake, we are not obligated to correct him, as this is similar to returning his lost item, which we are not obligated to do.