Is selling a phone in installments and then selling it for cash to another person, after informing the seller that I will sell it, and his saying that he will not buy it himself, considered a prohibited 'inah transaction, or does the sin fall upon him?

There appears to be no Sharia impediment to the validity of the aforementioned transaction. The buyer's failure to take physical possession of the phone does not affect the validity of the sale, as possession is considered to have occurred constructively through 'takhliya' (making the object available to the buyer). This is the view of many scholars, where possession is achieved by the seller designating the device and not preventing the buyer from taking possession of it. Furthermore, many scholars permit the buyer to appoint the seller as an agent for taking possession of the commodity, which is the apparent view of the Maliki school and the correct opinion within the Hanbali school.