Q: What are the guidelines of ona’ah?

A: The Torah commands: “When you sell to your fellow or purchase from the hand of your fellow, do not aggrieve one another” (Vayikra 25:14). Sellers and buyers are admonished not to deceive or cheat the other party of a fair market price. If an item was overpriced, the buyer can claim ona’ah; if an item was underpriced, the seller can claim ona’ah (C.M. 227:1). Chazal determined that ona’ah is calculated by a price differential of 1/6. If the differential was less than 1/6, we presume that the aggrieved party was mochel and has no claim. If the differential was 1/6, the sale remains valid, but the differential must be refunded or paid. If the differential was greater than 1/6, the aggrieved party can invalidate the sale. The 1/6 is measured both in respect to the fair market price and the price paid. Thus, if an item typically costs $60, the buyer is entitled to a refund of his overpayment whether he paid $70 (1/6 of the value) or $72 (1/6 of the price paid). The seller is entitled to additional payment whether the item was bought for $50 (1/6 of the value) or $51.43 (1/6 of the price). If the price gap was less than this, there is no claim of ona’ah; more than this, the sale can be invalidated (227:2-4).