The big question on everyone's mind is where the new third-party books are coming from. Here are my personal views:
* Many publishers suspect that the books are not actually new, but rather are used from various sources and are being sold as new. However, there hasn't been any proof that this is a widespread phenomenon. OTOH, there is practically no barrier to a seller making a "new" claim for a used book. The main disincentive is Amazon might punish them if it's discovered or enough readers complain.
* Review copies are being sold as new. I know two publishers who have received complaints about this from readers who purchased a "new" book from Amazon, only to discover that it was marked as an ARC or not for resale.
* Third-party sellers are getting their hands on overstock or returns from retailers that were supposed to be pulped or destroyed. I have heard of two businesses that have operated in this realm, but both appear to be legitimately purchasing overstock as opposed to buying copies that should have been destroyed.
* Third-party sellers are accessing wholesale channels intended for retailers or libraries. These sales are legit, but publishers get less money from them. See the Createspace thread below (first two pages) for a discussion of one case.
* Some of the new books are pirated. This is my personal view that is not widely shared, but I believe this is going to be a big problem going forward, based not only on two documented cases this year (Dave Burgess' pirated copy of "Teach Like a Pirate" and Bill Pollock's "Python for Kids", see links below) but also because it's such a widespread phenomenon elsewhere on Amazon. It's a matter of time before pirates discover it's open season for books, and more and more will rush in using cheap short-run digital or print on demand to own the buy box.
There may be other sources as well. If the books are used and are being sold as such, it's accessed through a wholesaler, or it's overstock sourced legitimately, that's not an issue. But publishers should be concerned if used books and review copies are being sold as new, or pirates are eating their lunch.
1. https://twitter.com/burgessdave/status/849039943561134080
2. https://twitter.com/billpollock/status/844030960333152256
3. https://forums.createspace.com/en/community/message/76847#76...
ETA: Wholesale channels bullet and "new" claims details