While Walmart Marketplace is growing fast, it’s struggling to gain traction with its relatively clunky seller system that has proven to be especially difficult for smaller and inexperienced sellers to navigate. Meanwhile, those familiar with the program say there are a lot of issues that still need to be worked out. “As a seller, you really should be more established, more experienced, have a team to handle Walmart, because it’s not as intuitive as Amazon,” said Pauline Shiu, marketing director at Zentail, which manages listings for third-party sellers.