CHICAGO - Between 2009 and 2014, Walmart’s Marketplace business, where outside merchants hawk everything from baby blankets to power tools, counted no more than six sellers, and was described by one expert as “in limbo.”

But what was treated as an afterthought for years has emerged as an important leg in the world’s biggest retailer’s long-term strategy to take on Amazon Inc, which it is battling for advertising and ecommerce dollars.