Baba Metzia 4:3-4

Baba Metzia 4:3

The following is considered an unacceptable level of overcharging and must be repaid: four silver maos from the 24 silver maos in a sela, which is one-sixth of the purchase price. The overcharged party can void the transaction for the amount of time it would take to show his purchase to a merchant or a relative (to assess whether it was worth what he paid). Rabbi Tarfon taught in Lydda that overcharging is eight silver maos to the sela, which is one-third of the purchase price. The merchants of Lydda rejoiced at this news (since this would give them larger profit margins on transactions that could not be voided by the buyers). Rabbi Tarfon then told them that the buyer has the right to cancel for the entire day. The merchants of Lydda preferred the way things had been before and went back to following the opinion of the Sages.

Baba Metzia 4:4

The rules of fraud (that a person cheated of more than one-sixth of the purchase price may cancel the transaction) apply to both the buyer and the seller. Just as fraud applies to lay people, it likewise applies to professional merchants (who can cancel a transaction if they receive too little for a purchase). Rabbi Yehuda says that this rule does not apply to merchants (who are responsible to know the value of their wares). The one who was shorted has the legal advantage: he has the option to cancel the transaction or to demand the difference between the purchase price and the fair market value.