What is the ruling on promoting and selling products on behalf of a wholesaler, where the promoter takes the difference between the wholesale and retail prices?

Your work as an intermediary between a merchant and a buyer for a known commission is permissible, as is working as an agent for a merchant in selling for a known wage, or agreeing with the buyer and then purchasing the goods and selling them to him. All of this is permissible. What is forbidden is selling goods before possessing them.