The question If a book doesn't have DRM does that mean it is OK to give a copy to my friend to read? has answers implying that without permission there are legal and ethical hurdles in giving away a copy of an ebook. In the United States, there are stores selling used books, which are under copyright.
As Harvard Law School buys back used text books, presumably with the intent of resale, it is probably safe to assume the action is not illegal, in the US.
It seems that selling or giving away your copy of a print book that is under copyright is a not a problem, so how is that different than doing then passing on your only copy of an ebook, assuming you delete the existing copy on your device?