What is the ruling on selling products that the seller does not possess, then purchasing them after the buyer requests them, knowing that the buyer is unaware of this?

There is no harm in creating a website and displaying products on it, but it is not permissible to mislead buyers into thinking that the products are owned by you in order to conduct a sales contract for them. This falls under the category of selling what you do not own, which is prohibited. This can be resolved by informing the buyer of the truth, that the item is not with you, but that you will procure it for them from its owner in exchange for a known commission.