Assalaamu alaykum. I would like to know if this business model is halal or not. I work for a print on demand garment workshop, and we work with users who can design their own design (a logo on a t-shirt for example) and then sell them to their customers. When they make a sale, we print the custom clothes and send it to their clients. In return, the owner of the custom t-shirt pays us for each order. We may also help them in setting up their e-commerce store (products etc...) to help them make sales. Is this permissible? Does it come under the 'Do not sell what you do not own' rule? May Allaah reward you.

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

In principle, there is nothing wrong with your work of printing designs on t-shirts regardless of the nature of the transaction between the store owners and their customers and whether it is an Istisnaaʻ sale contract (contract of exchange with deferred delivery applied to a made-to-order product), or a Salam contract (contract according to which the price of a clearly defined item is paid in advance at the place of concluding the contract and the sold item is to be received later), or a mere promise.

It is also possible that the transaction is a sale of an item that one does not own, but the basic principle is that the transaction between them is lawful. You are not obliged to investigate the nature of the transaction. If it came to your knowledge that any of their transactions involves a violation of the sharee'ah, then you should advise the concerned party to abide by the criteria of the sharee'ah.

Allaah knows best.