“Sometimes there are hidden obstacles to scaling—a lesson that eBay has learned in recent years. Like all marketplaces, the auction marketplace lent itself to natural monopoly because buyers go where the sellers are and vice versa. But eBay found that the auction model works best for individually distinctive products like coins and stamps. It works less well for commodity products: people don’t want to bid on pencils or Kleenex, so it’s more convenient just to buy them from Amazon. eBay is still a valuable monopoly; it’s just smaller than people in 2004 expected it to be.”