What is the ruling on a teacher (worker), a seller, or a shop owner taking a commission for recommending a specific product to a buyer, without the buyer's knowledge, especially given that this commission sometimes exceeds one-third of the product's original price and is only taken in certain areas?

There is no harm in a person being an agent and a broker for one individual, and receiving a commission for selling an agreed-upon commodity, provided there is no deception or misrepresentation to the buyer. What is based on lies and deception, such as the seller telling the buyer that the goods are unparalleled in the market, while knowing that the buyer does not know their true price, is forbidden due to the deception involved.