What is the ruling on selling products on a marketing platform as an agent for a supplier, while setting a higher price for the product, guaranteeing the buyer's rights, and with the supplier's approval and knowledge?

Your agreement with the product owner to market and sell their products is considered an agency for a known fee, which is permissible. It is permissible for the agent to take what exceeds the price set by the seller. For example, if the seller sets the price at five, and you add fifteen as commission, there is no harm in that as long as it is free from deception and fraud. There is no known limit to profit, and it is permissible by mutual consent of the two parties, for the Messenger of Allah, peace and blessings be upon him, said: "Indeed, selling is by mutual consent."