In this articleWMTwatch nowWalmart's online marketplace has become a key part of its strategy to grow profit faster than sales and better compete against its longtime rival, Amazon.As the largest U.S. retailer with more than 4,600 locations nationwide, growing sales online is also critical for its future.But a CNBC investigation found Walmart's digital boom came as it made it easier for third-party sellers to join and sell on its marketplace, a strategy that has come with a cost.Some consumers have received ...