Is it permissible to establish an online store for selling groceries and delivering them to homes with a profit margin for delivery, knowing that some goods may not be available with the seller at the time of order, and he purchases them after the customer's request, then delivers them to him?

It is not permissible to sell a commodity that you do not own or that is not in your possession, such as selling a specific item and then buying it later; this is forbidden by Sharia (definition). However, selling a commodity described in terms of liability (fi al-dhimma) and not as a specific, existing item, provided the price is paid at the contract session, or acting as a broker between a buyer and a seller for a known fee, is permissible. As for delivery services and charging a fee for them, even if contracted within the sales agreement, it is permissible, because combining a sale and a lease is permissible if the benefit is known.