How can one differentiate between agency in buying and selling what one does not own, and what is the ruling on agency in buying and selling and taking a fee for it?

There is a clear distinction between agency for purchase at a known wage and a person selling what he does not own. The agent buys the commodity for the principal from its owner in exchange for a wage for his effort, not as profit from a sale. The sale here takes place between the owner of the commodity and the buyer's agent. As for a person selling what he does not own, it is forbidden. However, agency in buying and selling is permissible, whether for a wage or without one, due to the need for it.