“One of Wal-Mart’s biggest challenges is that its investors demand profits, while Amazon has been allowed to run its retail operations at a near loss, or an outright one, as long as it keeps growing rapidly.... Wal-Mart has a market capitalization approaching $300 billion and a price-to-sales ratio of 0.6. Amazon is worth nearly $550 billion and has a price-to-sales ratio of 3.5. Which stock is more vulnerable?”