There's no such thing as a "used" digital file. That means it has to be a new file. So this would be...piracy? Because making copies of a digital file without compensating the owner of the rights to that file is piracy.
But this isn't what Amazon is doing. They aren't selling used eBooks. They're re-selling the license to use the software that is an eBook, as is suggested in the comments (the title is a wee inflammatory and simplified). To be clear: eBooks are software, and software is sold through usage licenses. When you "buy" an eBook you're actually buying the rights to access to that file.
So this means I could sell my license-to-view an eBook back to Amazon and they resell that license to someone else. This is virtually unheard of in the software industry (I think, from the anecdotal evidence I've seen--not so in the gaming industry though, where used software is resold all the time--at least when there's a physical copy of the disk. I don't believe it's common to resell digital seats to video games; I could be wrong). With software, it means I could resell my rights to Microsoft Word, say, back to Microsoft, and they could "resell" my seat back to someone else. Except, why would they do that? Why would a manufacturer buy something back to resell it? They wouldn't. But a reseller, like Amazon, might, because they're making money twice. But only a CONTRACT can ensure that the producer (Microsoft, or Tor books, for instance) get money on first sales and re-sales.
This, btw, stretches past eBooks into audiobooks and digital games and all sorts of software, I believe. I wouldn't be surprised if Apple or Microsoft were the first to sue; at the least they'll be watching this carefully because it affects their business too since really, software is software is software.
At any rate, I have little faith in a lawsuit, which I have no doubt will launch tomorrow. (Actually, I bet there's talk in lawyers' offices this very afternoon.)
This can be solved another way, in which everyone is happy and making money, from readers to Amazon to publishers to writers.
Amazon and publishers can write contracts to compensate publishers, and thereby writers, every time a license is sold and resold. Off the cuff, I think Amazon wouldn't like that. So the second option is that the publishing industry as a whole remove all products from Amazon, which could then become the bastion for all things self-published (and probably crappy content, because no writer worth her business-salt would be all down with re-licensing digital files without receiving compensation for it.)
Amazon will call this the same as re-selling paperbacks. It's not, because a paperback can wear out. A digital file doesn't. Plus, reselling paperbacks take some time to make the rounds in the market. They are really the very slightest competition to the same book in new, unused form; people who buy paper books often want to buy and keep their books. Garanteed, digital files for resale would appear almost simultaneously with the release of eBooks. It requires tremendous transparency, as Scalzi mentions, and Amazon has never been down with transparency.
This is a HUGE opportunity to make money, but Amazon should not be the only one making money off a license to view an eBook that is sold and resold. That is abundantly clear.
It's also a huge opportunity for Barnes and Noble to stand up and do the right thing by the market, by publishers, and by writers, thereby making a solid ally of the publishing industry rather than going on an all out offensive against it, which this clearly is. And it's important that someone make an ally of the publishing industry and keep it going. Because, honestly, without a nimble, professional publishing industry, even with the slight success of self-publishing, I believe quality books will go the way of teenaged angst-filled blogs.
Now, I'm not all anti-Amazon, actually. They are one of my publishers (Audible, which they own, published EXILE's aBook) and I buy some books from them, too. I like my Kindle app better than the others. But I'm thinking... thinking about easing to another app.
Mostly, it's clear Bezos does what big business in the US does; he does only what's good for Bezos and spins it into making customers and writers think it's good for us. Mostly that works; writers are notoriously poor businesspeople, myself included. And there's this one niggling thing: Amazon relies on cheap content to fill their devices. So does Apple, btw. (exhibits A and B: the millions of cheap cr-apps on iTunes, and the price of my book on Amazon vs other places).
Withdraw the content, Amazon fails. Publishers have the ultimate weapon in this potential war. Now, to see if they'll wield it.