What is the ruling on working through a platform for selling medical supplies on a commission basis, where the marketer does not directly own the product but presents it as if it were the main store, with the possibility of adjusting the commission, and what are the necessary modifications to make this transaction permissible?

The questioner is acting as an agent for the platform, and it is permissible to take a commission or fee from the seller for the agency in selling. Agency is permissible with or without a fee, and it may be for a wage or without a wage. The wage becomes due when the work is performed. The platform sells what it owns and does not sell what it does not own. If the commodity is specified, absent, and owned by the seller, then this does not fall under forward selling (bay' al-salam).