What is the ruling on selling goods in installments—at a rate of 25%—that I do not own, where I purchase them after the customer requests them, and then sell them to him in installments? Is this considered selling what I do not possess? And what is the way to make this transaction permissible?

It is not permissible to sell a specific commodity before possessing it, due to the Prophet's (peace be upon him) prohibition of selling what one does not own. For this sale to be legitimate, an agreement is made with the buyer to procure the commodity, then purchase it, and then sell it to him. A deposit can be taken as a guarantee of serious intent to buy. The seller is not entitled to deduct anything from the deposit except for the actual damages incurred if the buyer reneges on his promise. As for purchasing cheap products and selling them at a higher price by installment, there is no harm in that, because time has a share in the price.