There could be an opportunity for a competitor depending on how some Amazon issue play out over the next year or so. A lot of commentors are focused on the tech (which makes sense, being HN and all), but Amazon has some issues with counterfeit products, unequal product commingling, and unreliable third-party sellers.

Right now those issues are mostly side-line grumpling, but I could imagine a few big incidents could possibly blow up into a substantial reputation issue. The unknown, of course, is can/will Walmart do any better.