Tech-a-nica-lica-ly, the difference between "buying" an e-book and "licensing" an e-book are the terms of acquisition. Truth be told, about 25% of license agreements are rehashes of federal laws (don't make copies and give them away, don't make copies and sell them, etc) that most people aren't bright enough to realize are laws despite the inherent comprehension of exhanging wealth for labor when physical objects are concerned.
Because many consumers are stupid and a few software writers were incredibly smart/greedy, they started selling things on a license. The licenses made sense to a point; you pay for 1 thing, you can use the thing in one place. Since software could be installed multiple places, making it a license vs. a purchase meant you had legal recourse when someone installed it willy-nilly.
I'm cool with that, really. It means you've got two hammers to hit people with; civil contract violation and criminal offenses.
The trouble came when the licenses became unreasonable. Non-transferrable licenses, for instance. I'm sure the justification was that you can't be *sure* the seller didn't keep a copy for themselves, but that's really just laziness. They make it illegal to transfer the licenses so they can just track down *all* resellers instead of just the unethical resellers. IIRC, in some states the non-transfer limit of a software license is illegal, hence the thriving used PC game market. Even DRM'd games can be resold since the DRM protects against copying the base media.
Unfortunately you can't have a thriving used e-book market because it is media-less. The DRM usually requires an authenticating server so local authorities can't invalidate the license. I'm waiting to see if these illegal DRM licenses force the dealers to not sell to citizens in those states that don't accept non-transferrance clauses.
I personally will *never* pay for an e-book license. My job may pay for such, but I won't. I *will* buy e-books. Do all the time, but from publishers who don't make it onerous. I have 5 functional computers in my house and you want me to register me e-book to a single machine? Worse, you want me to register it to a single OS installation?!?
That amounts to buying an album, but only being allowed to play it on the current stereo in your current car. New car? Sorry, no album. New car stereo? Sorry, no album. Home stereo? Sorry, no album. Bite me, I'll buy paper and be fairly confident that you're making *less* money on it than if you'd sold it in a reasonable electronic format.
I think in an average year I buy about 10-12 e-books, plus I'm a paid writer for an online magazine so I'm on both sides of the consumer fence here and not some neo-luddite.
Now, do I think the used e-book market will thrive? probably not. Ebay won't bother; there'd be too many legal hassles. Stores? Nahh, they won't trust it. And buyers won't have any confidence that what they're getting is a complete copy with all add ins. (meaning a text PDF as compared to scans in a PDF. I love e-books for search functions and being able to copy/paste text into my game note and hand-outs.) So I figure the used e-book market will be a non-existent entity. After all, as long as the publisher is still in business, there's no reason the book should ever go "out of print."