Or just regrouping.
Or, bottomline, closing a door to extra sales for online booksellers.
Finding, listing and selling old textbooks probably will be a lot harder now that the Amazon Marketplace is adding new restrictions, as noted at this website: The Book Flipper
If your business plan includes reselling used textbooks, read this article and start determining if sourcing textbooks is still worth the risk.
I used to be able to post DVDs and music CDs for sale on Amazon; but now, the ones I find and flip online go straight to Ebay. (Note: they are selling pretty well there, and for less money in fees too).
My thought is that Amazon is making these restrictions not so much to hurt booksellers or limit competition, but, rather, to avoid legal threats of copyright infringement or claims of counterfeiting (which, sadly, is a real thing).
They've already restricted most sellers form posting books in "New" condition unless you can produce invoices direct from a publisher (and you'll need to be an authorized dealer in most cases to make these purchases.)
In any case, our advice for now is to simply protect yourself from investing too heavily in textbooks so you don't get stuck with un-sellable stock.
Better to spend your money on good used non-fiction in ice categories -- the types of good used books that have long-tail appeal and sell for good margins online.