You are legally allowed to buy a book and resell it. If you look, many of the legitimate offers for books on Amazon that are "from the long tail" of content are

You are legally allowed to buy a book and resell it. If you look, many of the legitimate offers for books on Amazon that are "from the long tail" of content are coming the inventory of small book stores, both used and new. The issue is not that third parties are selling a book at all but that the book they are shipping is a third-party low-quality re-print of the book in question instead of an official copy, something you wouldn't know until you receive the book.