What is the ruling on buying goods in bulk and leaving them with the seller for marketing and sale in exchange for a commission, with the buyer also marketing them and the customer receiving them from the seller's shop?

It is not permissible for someone who has bought something to sell it until they have taken possession of it and removed it from the original seller's premises. This is based on the Prophet Muhammad's (peace be upon him) saying to Hakim ibn Hizam: "If you buy something, do not sell it until you take possession of it." He (peace be upon him) also forbade selling goods where they are purchased until traders transport them to their own places.

Therefore, it is not permissible to sell something you bought online while your item is still with the seller, or to send the customer to pick it up from him. Rather, you must take possession of it and remove it from his premises before selling it.

It is also not permissible to authorize the seller to take possession of the item on your behalf and then give it to the customer, so as not to nullify the prohibition against selling something before taking possession of it. The Islamic Fiqh Academy has prohibited organized tawarruq, where a bank sells an item to a customer and then acts as his agent in selling it to another party. This is considered akin to the prohibited inah, and because the bank's commitment to sell through agency makes it superficial, and because it leads to a violation of the conditions for lawful possession.