What is the ruling on money acquired as a result of secretly trading—without the knowledge of relatives—on the price difference when purchasing their necessities?

It is permissible for you to buy the commodity for them at a price lower than that offered to them and to take the difference for yourself, provided that you inform them of this and they agree. Otherwise, it is not permissible to take a commission without their knowledge and consent, because an agent is not allowed to take the difference for himself without the knowledge and consent of his principal.