What is the ruling on purchasing an iPhone in installments from someone who goes to buy it from a specific store after the buyer has chosen it, and then sells it to him in installments?

As long as the second party will buy the device for himself first, then sell it to the one requesting the purchase after it enters into his possession and guarantee, there is no harm in that. Nor does it affect that he will sell it in installments with a profit, for time has a share in the price.