Is my method of displaying clothes with unique designs on an e-commerce website, by dealing with a print-on-demand factory, considered selling what I do not possess, knowing that I inform the customer that the manufacturing is done through the factory?

If there is no connection between your contract with the client and the manufacturer other than the client's knowledge of the manufacturer's name, then there is no harm in that. This is similar to the permissible parallel Istisna' (manufacturing contract). Its form is: the buyer agrees with you to manufacture a specific good with precise specifications, and pays all or part of the price, or defers it until after delivery. Since you do not have the manufacturing capability, you enter into an Istisna' contract with another company under the same conditions and at a lower price. The Islamic Fiqh Academy has permitted the Istisna' contract under certain conditions: specifying the type, kind, quantity, and required characteristics of the manufactured item, and setting a deadline. It is permissible to defer the price or pay it in installments, and it is also permissible to include a penalty clause in the contract. The Sharia (definition) Standards state: It is permissible to conclude an Istisna' contract even if the institution does not already own the item to be sold, and it is binding if its conditions are met: specifying the type, kind, quantity, and required characteristics of the manufactured item, knowing the price, and setting a deadline. It is not permissible to stipulate a waiver of defects. It is valid for items that involve manufacturing, and it is permissible to contract for the manufacture of items with special specifications even if there is no similar item in the market. It is permissible to defer the price or pay it in installments, and it is permissible to link installments to precise completion stages. The manufacturer's liability is discharged upon delivery. If the manufactured item does not conform, the buyer has the option to reject it or accept it with a settlement. It is permissible for the institution, as a buyer in an Istisna' contract, to enter into a parallel Istisna' contract with another party, provided there is no link between the two contracts.