10. One who sees a simpleton or naïve fellow about to make a purchase, from a cunning merchant – who usually fools people with dishonest weights, or with prices that are inflated more than a sixth of the regular price – he is obligated to warn the fellow not to make the purchase, even if the fellow has already agreed to the purchase.

So too, if he sees a merchant trying to cheat his client into buying shoddy merchandise, he is obligated to warn him not to buy, however he must make sure that all the conditions written above are met [A. He is certain of the harm. B. He must not exaggerate. C. His intention is for a constructive purpose. D. There is no other way to effect a solution. G. The report will not cause the subject any harm, which is not sanctioned by Torah law].