It's pretty much impossible for Amazon to tell whether an item is a licensed use of a copyrighted creative work or not. It's perfectly legal to resell work that was legally bought with no additional permission from the creator, for instance. (It is pretty clear from a human analysis here that the seller is infringing copyrights by making prints of digital images found online, but it would be pretty much impossible to create some sort of filter that would block these products from being offered on the platform).
From the thread, it looked like the support representatives engaged with the person complaining on Facebook, reported some items had been removed in the past based on takedown requests. That's pretty responsive, all in all. Maybe the artist could enlist some additional help discovering and helping him and other affected artists report for takedown the items that this seller is offering so they'd eventually . Or they could gather some funds and sue Amazon.
From the thread, it looked like the support representatives engaged with the person complaining on Facebook, reported some items had been removed in the past based on takedown requests. That's pretty responsive, all in all. Maybe the artist could enlist some additional help discovering and helping him and other affected artists report for takedown the items that this seller is offering so they'd eventually . Or they could gather some funds and sue Amazon.