The Laws of Commerce and Trade
1. Great care must be taken not to deceive or cheat a colleague. Whoever
cheats or deceives a colleague violates a Torah prohibition. This applies
whether the seller deceives the purchaser or the purchaser deceives the
seller, as [Leviticus 25:14] states: "When you sell something to a
colleague or buy something from a colleague, do not wrong each other."
[Shabbos 31a] relates that this will be the first issue about which a
person will be questioned [by the Heavenly tribunal] in his ultimate
judgement. He will be asked: "Have you dealt and traded honestly?"
2. Just as one is forbidden to cheat a colleague with regard to a sale, so,
too, there is a prohibition against cheating with regard to hired work,
contracted labor, and exchange of currency.
3. A person who trades faithfully need not worry about the prohibition
against cheating a colleague. What is meant by "trading faithfully"? A
person who says: " I have brought this article for this amount and I want
to make this much as a profit." Even if he was cheated when he purchased
the article - though a person who was cheated has no right to cheat a
colleague - nevertheless, the above mentioned practice is permitted. He
clearly tells the purchaser not to consider the worth of the article, but
the amount which he had paid for it.