Is it permissible to photograph products and advertise them for sale before acquiring them, knowing that the purchase is not made until buyers are found, and does this constitute selling what one does not own? If that is permissible, is it then permissible to return them after purchasing them if they are not sold to a buyer?

If what transpires between you and the customers is merely an agreement to sell, pending your purchase and ownership of the commodity, then the transaction is Islamically permissible. If you buy the products without stipulating an option (for cancellation), then you do not have the right to annul the sale and return the products to the seller due to the customer's reneging on completing the transaction, unless the seller agrees to the cancellation.