I wanted to buy some mini marshmallows recently, so I went on Amazon. Perhaps because of their resemblance to packing material—light, bulky, ubiquitous—I figured they’d be cheap. But when I found the most popular brand, not only did the marshmallows cost twice what I’d pay at my local store, but the price had skyrocketed overnight.
Just beneath the placid surface of a typical product page on Amazon lies an unseen world, a system where third-party vendors can sell products alongside Amazon’s own goods. It’s like a stock...