> 2.8 billion people live on less than 2 dollars a day. A second hand smart phone might go for 20 dollars. So its a few weeks worth of savings.

Ten weeks income, and someone who lives on 2 dollars a day can't afford to save 100% of their income to buy a smartphone.

And how much would used smartphone prices increase if a billion people wanted to buy one?

Sharing or renting makes more sense, but in that case, why not share the cost of a data plan?