Does the customer have to pay the full price of the goods on the basis that it is a Salam sale, or are we intermediaries between the customer and the manufacturer, and should we inform him of the factory price and our commission percentage?

This type of transaction is not a salam (forward) sale. If the agreement with the customer is binding, either through a contract or the receipt of payment, then it is forbidden, based on the Hadith (definition): "Do not sell what you do not possess." However, if the agreement is not binding, then it is permissible after purchasing and taking possession of the goods to sell them to the customer. It is also permissible to take an agreed-upon percentage in exchange for providing the goods, acting as an intermediary.