The Authors Guild's argument is that authors don't get any compensation if someone purchases a used book; only the seller and Amazon.com make out on the transaction. So when amazon.com makes it easier for consumers to buy and sell used books which could also be purchased new --- at a more expensive price, of course --- it hurts the authors. The Authors Guild and the Association of American Publishers would prefer that Amazon.com only allow their users to sell used books if they are out-of-print books.
Well, excuuuuuuuse me! I can understand that authors need to eat, and send their kids off to college, and all those good things. But if a book wasn't good enough for me to want to keep it, why shouldn't I be able to sell it? Using the same logic, the Authors Guild should logically be against public libraries. After all, people who use libraries can (oh horrors!) read a new book without having to pay for it!
This really goes to show the fundamental tension between content providers and consumers. If you take the Authors Guild position to its extreme, you'd think that they would much prefer that bookreaders purchase books from bookstores, and if they didn't like it, that they throw it into a landfill rather than resell it or give it away, or lend it to a friend. After all, all of these activities compete with new book sales. Fortunately for us, the doctrine that the owner of a book is allowed to do all of these things is fairly strongly encoded into law --- which is why all the President of the Authors Guild can do to write a whiny letter to Bezos asking him to please don't do this.
And thus we see the danger of the positions espoused by the Software Publisher's Association, and UCITA. Not only do they wish to take away our rights about what we can and can't do with software --- including rights which common sense would dictate are perfectly permissible in the case of the physical world, such as selling or loaning a book to a friend --- but their actions have emboldened folks such as the Authors Guild and the Association of American Publishers to try to take away rights which we always have had in the physical world. After all, if the software vendors can restrict what you can do their software, why shouldn't a book publisher be able to restrict what you can do with their books?
Fortunately, most book publishers don't have as much money to throw around as Disney, so they probably won't be able to purchase enough Senators to change copyright law to suit their purposes. But when thought patterns of the SPA have started infecting traditional book authors --- who really should know better --- it's obvious that we're living in dangerous times."