To varying degrees, Amazon displays all these characteristics, and by its breadth and complexity, confounds traditional antitrust analysis. What began as an online book retailer now sells just about everythingunder the sun— not just online but, more recently, also through physical stores and pickup depots. In hundreds of high-volume categories, Amazon is not only a retailer but also produces its own branded line of merchandise.

Through its online marketplace, customers can buy from Amazon but also from millions of competing retailers who typically pay a 15 percent to 20 percent commission and now account for half of all unit sales on the Amazon platform — and a quarter of Amazon’s total profits. Many of these “third-party sellers” pay additional fees to store their inventory in Amazon warehouses, use Amazon robots and personnel to fulfill customer orders, or rely on Amazon to deliver their goods to customers across the globe through its fleet of 25 planes and 4,000 trucks or its deeply discounted delivery contracts with UPS and FedEx.

This remarkable business machine, offering 350 million items for sale, is fast approaching the point where it can claim nearly every household in America as a customer.