Walmart just took a big step in its move to break Amazon's control over third-party sellers and is officially letting sellers sign up for its fulfillment service (AMZN, WMT)

· Walmart started inviting third-party sellers to its new fulfillment service on Tuesday, following a trial run over the past few months.
· The new service, called Walmart Fulfillment Services, helps store and ship products for third-party merchants selling on its site, and provides other services like returns and customer...