Halachic issues with buying an item for cheap when owner doesnt know true value

Question

What are the halachic problems that arise when one buys an item at a discount but the owner is unaware of the true value? This can apply to a yard sale where one buys a coin that they know is worth thousands and the owner is selling it for pennies. Does one have to tell the owner of the true value?

Another such case which I have encountered is going to a book sale and there is a $1 table, but on that table there is a book that is worth $100. Is one halachicly obligated to inform the store owner?

Just to note why these cases may be different from other price mistakes is that it is known that there may be rare stuff at a yard/book sale.

Answer 1

Upon investigation, I found in the sefer Mishpetei HaTorah from Dayan Tzvi Shpitz Chelek 2:26 a similar type case.

He relates that Reuvein found an old copy of the Rambam in genizah. He then sold it to an antique seller for 1000 shekalim. The antique seller then noticed after the sale that there are handwritten notes on the side of the sefer with the signature of the Rama. Now the sefer is worth 50,000 shekalim. The question he was asked was this a mekach taus. Dayan Shpitz answered that antiques, in general, do not have set prices, but fluctuate from time to time and therefore the halachos of onaa do not come into play. However, he does note that if this sefer would sell for 10,000 shekalim minimum in any place, then since it was sold for only 1000 shekalim then the seller can claim onaa.

Then he discusses a different twist on the case. That if the book has other parts to it that are not associated with the book. Like if one found a ksav yad in the book which by itself is worth money and the seller did not know of its existence then the buyer can keep it with no complaints. However, he notes that if the seller got that book from yerusha (inheritance) then the buyer would have to return the item.

(He brings the sources to his psak on the bottom of the pages, it is lengthy. It's on pp. 127-131)

In addition , I found that the L'Horos Nasan 7:126 speaks of a case where a seller sold a ksav yad of a gadol for a minimal amount and then found out that there people who would be willing to pay more for it ,is that a case of onaa and would cause the sale to be void? He goes through a bunch of different sugyos and concludes that if there people who are willing to pay for the item then there would be onaa. He also says a big chiddush that the fact that today it is easy to reach people all over the world then the market encompasses the whole world .

Answer 2

Here's a short list of possible problems:

1. The prohibition of Hona'ah (אונאה - fraud) - selling or buying for over 17% of its real price either mistakenly or on purpose. In certain cases, it can be so serious it invalidates the deal and causes a big mess called מקח טעות. Technically, there's no fraud for real estate, personal belongings, slaves, bills and more.

It is a very common misunderstanding that (especially those who don't study Choshen Mishpat 227):

"אסור להונות את חבירו בין במקחו בין בממכרו ואיזה מהם שאינה בין לוקח בין מוכר עובר בלאו:

כמה תהיה אונאה ויהיה חייב להשיב שתות (1/6 - 17%) בשוה.
כיצד הרי שמכר שוה שש בחמש או שוה שבע בשש או שוה חמש בשש או שוה שש בשבע ה"ז אונאה ונקנה המקח וחייב המאנה לשלם האונאה ולהחזירה כולה למתאנה: "

"היתה האונאה יתירה על השתות כל שהוא כגון שמכר שוה ששים בחמשים פחות פרוטה בטל המקח והמתאנה יכול להחזיר החפץ ולא יקנה כלל אבל המאנה אינו יכול לחזור אם רצה זה וקבל/

4. גזל - steal. Possessing an object of others' without properly paying as the result of the above is גזל and is really bad.

5. לפני עיוור (before a blind man) - the prohibition of deceit. One can not give a bad advice or name the wrong price when asked.

6. ואהבת לרעך - one is "prohibited" from causing others what he wouldn't want for himself.