To make a fortune selling masks on Etsy, you needed three things. First, you needed to be familiar with the garment industry — ideally, already working in it. Second, you needed to be in or around Los Angeles. (Sorry, New Yorkers.) And third, you needed energy, because shipping tens of thousands of masks a month with a tiny crew meant working late into the night, then getting up early the next morning to do it all again, sometimes, while caring for kids in between. For those who went all in, the effort was worth it: some mom-and-pop Etsy shops became...