What is the ruling on selling products online at retail prices that differ from wholesale prices, without the intermediary disclosing to the buyer that he is merely an intermediary?

Your work as a broker for the seller is permissible, and there is no harm in taking a fee for it. It is not a condition that the buyer knows you are acting as a broker for the seller. Rather, it is a condition that the seller, from whom you take the fee, is aware.