Long story short - for clearanced items, Amazon's new gating policy is making it more and more difficult to grey market (legit, new, but not manufacturer authorized) products. Amazon's set up a system for vendors to police their excess, and they keep internal metrics on how many items are being sold through third party vendors... and once that number/dollar is high enough, they outreach to the vendors themselves and do a sales pitch so vendors can be active on their platform.
For new items being arbitraged by a third party between like this Walmart->Amazon thing, Amazon basically sits on the sideline until the product accumulates enough sales, then outreaches to the vendor themselves. Basically the arbitrageur is doing market discovery for Amazon, and at the point where it's a particularly profitable item Amazon does their bizdev direct with the manufacturer to bring efficiency into their own marketplace. Really clever and virtuous system, to be honest.
Most arbitrageurs now go for the private label option (buy unbranded items and rebrand with their own trademark) to avoid this, but once their product is popular enough to warrant it, Amazon responds by having their buyers clone an AmazonBasics version of them item. (See prior discussion: https://news.ycombinator.com/item?id=11533973)