NEW DELHI—Amit Rajput, who runs a counter selling iPhones in a busy electronics shop here, cuts a lonely figure. He is lucky to sell one device a day, compared with the 10 or more smartphones his colleagues at desks for Samsung Electronics Co., Nokia Corp. and China’s Oppo sell daily in the same store.

As customers walk past his display, he recalls a different time in 2013 when he sold as many as 80 iPhones a day. Now most people want to pay less than $300 for their devices—a fraction of what Apple Inc.’s newer models cost.

...