This is the Amazon Marketplace, where anybody can sell just about anything right alongside Amazon's own wares. Unlike eBay, where each vendor maintains a separate listings page, Amazon tidily groups its Marketplace sellers by item, hiding away the inferior offers, to showcase the best deals up front. (In seller parlance, landing the number-one spot is called "getting the buy box.") What looks so clean on your screen obscures the messy and massive jungle of the Marketplace: There are now more than two million sellers on Amazon. While the Seattle-based giant still sells the most popular items on the site itself, Marketplace sellers now ship nearly half of the products--about two billion items each year, all told--and those sales are growing twice as fast as Amazon's, according to the consultancy ChannelAdvisor. The Marketplace started in 2000 selling used books. In 2016, it's a retail phenomenon as significant as any in the past 50 years--together these sellers ring up what ChannelAdvisor estimates to be $132 billion in sales each year. That's more than Walmart sold in 1997. Yet we know so little about who they are. ... Pharmapacks, notched $31.5 million in revenue in 2014, which made its three-year growth rate 3,035 percent, good enough to earn it the 115th spot on the Inc. 500. By the end of 2015, its annual revenue was $70 million. Vagenas proudly told me the company was on track to do $140 million to $160 million in revenue in 2016, the vast majority coming from those platforms (and around 40 percent from Amazon). ... Inventory often stays in their warehouse only for a few hours before going right back out the door. The business is less like traditional merchandising than it is like a commodities trader from a bygone era, buying and selling well-known goods and turning a profit on each transaction.