Is it permissible to conduct a transaction of selling a product owned by a store in the city, by offering it online for a higher price, and when a buyer requests it, it is first purchased from the store, then taken possession of, and then delivered to the buyer for them to pay its price?

If a sale contract is concluded with the buyer before acquiring the commodity, then this is impermissible. However, if it is a non-binding promise, and then you acquire the commodity and sell it to him afterwards, this is permissible. The resolution of the Fiqh Academy stipulated the permissibility of a binding promise in a Murabaha sale, provided that the option (of cancellation) is available to both parties who made the promise. Otherwise, it is not permissible because it resembles the sale itself, and in that case, it is stipulated that the seller must own the sold item, due to the Prophet's (peace be upon him) prohibition of a person selling what he does not possess.