Is the money earned from selling a used device to a customer, after buying it from a broker for a certain amount, adding a commission to it, and selling it to the customer for a higher amount, permissible or forbidden?

For a commission to be permissible in brokerage, the person from whom it is taken must be aware of it, because brokerage is a jua'la (a contract for a reward for a specific act) that requires an offer and acceptance. Taking it without their knowledge is consuming wealth unjustly and is considered a major sin. It is not permissible to deceive the buyer by informing them of an untrue price.