Amazon also has the Amazon Marketplace, which allows third-parties to sell goods through the Amazon website. A third-party could just sell those Macmillan books

Amazon also has the Amazon Marketplace, which allows third-parties to sell goods through the Amazon website. A third-party could just sell those Macmillan books. And there would be nothing Macmillan can do about it since certain jurisdictions protect the rights of third-parties to resell items, like books. So a savvy Marketplace user would realize the potential of stocking Macmillan books and selling them through Amazon to cater exactly to those users who want them but can't get them through Amazon. Amazon would be more limited on the digital front, since that's a much trickier area, legally.