What is the ruling on selling products online without possessing them, where they are displayed in another store at a price higher than the original purchase price, and the original seller ships them directly to the buyer, while the difference is retained? Does the ruling change if the original seller is informed in advance?

It is not permissible to sell a commodity that the seller does not own at the time of the contract, due to the prohibition by the Prophet, peace and blessings be upon him, against selling what you do not possess. To avoid this prohibition, a promise to purchase can be taken from the buyer, then the seller buys the commodity and it enters into his possession, after which he sells it to the buyer, with the possibility of taking a down payment. Alternatively, the seller can act as an intermediary between the seller and the buyer by taking a commission from one or both of them.