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.

  • Re the 2nd paragraph - I don't see why there would be any obligation, unless it is extremely obvious to you that the owner is clueless. Even then, that may be a nice gesture. Once someone owns something, he may sell or dispose of it according to whatever conditions he wishes, with certain exceptions on specific objects. (E.g. you can't sell your share of Olam Haba; I don't think you can sell a shul to a Gentile; likewise, prob. with a Sefer Torah.) – DanF Nov 8 at 20:03
  • I also don't think there is a problem if the owner might be overpricing something if he doesn't know an items current price. People are allowed to make profit. You don't need to buy the item. – DanF Nov 8 at 20:06
  • בכדי שיראנה לתגר – kouty Nov 8 at 20:06

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)

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/

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

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

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

  • Re buying and selling - I am unaware of any problem with someone profiting from selling a tangible item. All businesses do this, as they need to cover overhead costs beyond the actual cost of the item. (E.g. a candy bar may cost the buyer 5 cents each. How could he sell it at cost and still cover delivery, inventory, wages and other expenses to keep him in business)? #2 puzzles me. How is he stealing? #3 - where does this come in? Isn't there a rule of "buyer beware"? Shouldn't a buyer be able to comparison shop? There's nothing forcing the customer to buy anything. – DanF Nov 9 at 18:39
  • @DanF Are you familiar with הונאה at all? 1. That's exactly the prohibition of a fraud either by upping the price for the seller or buying cheap for the buyer. 2. If the price is more than 1/6 of the "real market price" (needs a definition) the deal is off automatically and the buyer holds the item unrightfully, meaning it is a גזל unless the seller admits that he knows the real price. – Al Berko Nov 10 at 18:55
  • 3. It is מקח טעות as he thought he was selling a regular penny and it wasn't so the deal was never valid. The Gemmorah calls it עד שיראנו לתגר - the seller must show it to a valuer to know the true price, otherwise, it is a steal. – Al Berko Nov 10 at 18:56
  • @DanF Please read sefaria.org.il/… – Al Berko Nov 10 at 19:09

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