Be careful jumping on the Walmart train. Their online marketplace has recently started including third-party sellers as well. I'm not sure how they police them or if they're any better than Amazon, but you're not implicitly free from these concerns just by going to Walmart.com instead of Amazon.com. I would
guess that Walmart doesn't commingle stock from third-party sellers, since it's such a bad idea, but I don't know.
Jet.com also does third-party sellers, but I hear they are very strict and work hard to make sure that all their sellers are authentic.
Amazon has made itself the de-facto e-tail platform. We're already seeing a Google-like effect from them, where if Amazon is not happy with you, your business can be destroyed overnight. There are consultants similar to SEO consultants who try to ensure that your products always "win the buy box", some of which is certainly done through attempting to hit competitors with false negatives as you've described, much like the negative links that competitors try to register against one another to hurt their Google ranking.
I really hope that some of the other players in this game can get together and do something to stop another monopolistic pre-eminence. Between Shopify, ShopRunner, Walmart, and other brick-and-mortars who are jealous of Amazon, it should be feasible to create an Amazon-ish experience that will at least keep one or two other big competitors in the game. Let's do it before it's too late!