Shemiras Halashon Halachos
Hilchos Rechilus, chapter 9, section 10 I will write one example to make things clearer; others appear at the end of the book. Shimon
goes into Reuven's shop to buy merchandise and one knows that Shimon is gullible and that
Reuven generally cheats such people. One must warn Shimon of this even if he already agreed
to buy from Reuven. This applies even more if one saw that Reuven is actively cheating him (by
saying that a certain product is very high quality when it is not) through weights, measures, or
prices. All conditions of paragraph 2 must be observed.
(Comment: Regarding price, Reuven must charge six coins for something worth five. If the
difference is less than a sixth it may be that one should not tell Shimon -- see Be'er Mayim
Chayim. With weights and measures this does not apply).