Hilchos Rechilus, chapter 9, section 12

a. He must not make the story or the wrong bigger than it is.

b. His goal must be to help the buyer, not to enjoy the seller's shame — even if he knows almost for sure that the seller cheated the buyer. This point also includes something else: he must judge for himself that some good will come from telling. But if the buyer is a person who does not usually make claims and argue, and it will only hurt him to hear he was cheated and make him hate the seller, there is no need to tell him the truth. In a case where the buyer cannot cancel the sale, it is a mitzvah to praise the purchase to him, and this is not against "Keep far from a false matter" (Shemos 23:7).

c. If he thinks he can rebuke the seller so that he gives the buyer back the overcharge, he should rebuke him instead of telling the buyer.

d. If he can bring about the same good some other way than telling the buyer, he should not tell.

e. The person he tells must not be a talebearer. If talebearing is his nature, he causes him to stumble, because the buyer will tell the seller that so-and-so said the goods are bad or not worth the price. So he causes him to stumble in the ban on rechilus. Still, it seems that if he can warn the buyer not to reveal his name, and the buyer will listen, he should do so.