Over the last few years, Walmart's been working to leverage its brick-and-mortar empire and grow into a major player online. It didn't take long for the world's biggest retailer to build a massive digital marketplace with hundreds and millions of products and thousands of third party sellers. But Walmart's digital boom has a little known and much darker underside where some sellers steal the identities of legitimate companies so they can pedal counterfeit and sometimes dangerous products to unsuspecting onl ...