The Torah in Parashat Behar (25:14) commands, “Al Tonu Ish Et Achiv” – that we may not take advantage of our fellow when engaging in commerce. This establishes a prohibition against charging or paying an unfair price for merchandise without the other party’s knowledge, a law known as Ona’a. The Torah speaks in this verse of a situation where one purchases “Mi’yad Amitekha” – literally, “from your fellow’s hand.” The Sages inferred from this phrase that the laws apply only to Metaltelin – moveable articles, that are actually handed from the buyer to the seller – and not to Karka – real estate. More precisely, the concept of Ona’a applies to real estate transactions, but the laws are less strict in regard to these transactions than they are when moveable merchandise is sold. The reason is because people are more willing to agree to a high price for a piece of real estate, which they will keep permanently. Although one is not allowed to overcharge for Karka, nevertheless, if he did, the sale is final, as we presume that the buyer accepts the transaction despite the unreasonably high price he paid. Explaining the reason behind this command, the Sefer Ha’hinuch writes, very simply, that it is unethical to take people’s money deceptively, charging or paying an unfair price without their realizing that the price is unfair. Moreover, the Sefer Ha’hinuch adds, in order for society to function properly, people must commit not to cheat one another. If people all agree to deal with one another fairly and honestly, then they can conduct their affairs in a smooth, workable manner. But if people deceive one another, then commerce cannot be conducted functionally, because nobody will trust his fellow. The prohibition of Ona’a applies to both the seller and the buyer; the seller may not unreasonably overcharge, and the seller may not unreasonably underpay. Meaning, if the seller is unaware of his merchandise’s market value, and so he undercharges, the buyer – who is aware of the market value – is not permitted to pay the low price. He must pay a fair price for the merchandise he purchases. If a seller overcharged by the amount of a “Shetut” (literally, one-sixth), such as if he charged seven units of currency for merchandise valued at six, then the sale is valid, but the seller must repay the difference. The same applies if a buyer paid six for a piece of merchandise valued at seven. If the difference in price was less than a “Shetut,” then the sale is valid and no money is returned, as it is accepted that merchandise will sometimes be sold for a price slightly higher or lower than the market value. If the difference was more than a “Shetut,” then the cheated party has the right to void the sale. The other party, which overcharged or underpaid, does not have the right to void the sale. If, for some reason, the party who committed the Ona’a regrets the sale, despite having overcharged or underpaid, he is not entitled to void the transaction, as this right is given only to the victim. The victimized party can claim Ona’a only within the amount of time which, in the estimation of Bet Din, is needed to determine the fair market price. If the victimized party does not bring a claim until after that duration of time has passed, he may no longer claim Ona’a and have the transaction annulled. The laws of Ona’a apply only to commercially-sold merchandise. When people sell their belongings privately, it is accepted that they do not charge based on market prices, and thus buyers realize that they might be charged a significantly higher price. These laws pertain only to the item’s price. When it comes to an item’s weight or measurement, even the slightest inaccuracy voids the sale. If the merchandise was misrepresented as even marginally larger or heavier than its actual size or weight, the sale is annulled. One violates Ona’a only if he misleads the other party. If the seller is upfront with the buyer, and makes it very clear that he is charging a price