A few years ago, I visited a dusty warehouse selling second-hand clothes in Cotonou, Benin. In the back, behind bundles of used Canadian T-shirts, were two pallets of unboxed solar panels. I assumed they were destined for the roof. One of the employees told me otherwise. “Our boss sells them to his customers across the border,” she said, referring to Nigeria. “They use them for water pumps on the farms.” A few minutes later, the boss showed up and told me he expected second-hand solar would soon be a bigger business than the centuries-old, multibillion-dollar used-clothing trade...