Legally, in the US, the First Sale Doctrine. http://en.wikipedia.org/wiki/First-sale_doctrine

For a start, amazon reserve the right to revoke your account take away all your ebooks!

And then the lending provisions. You could lend your kindle to someone (giving them all your books rather than just one). With a paper book you can give the book away or lend it to people in a serial fashion for however long you want. The tech is clearly there (single, two week lends) but the terms under which it operates are far more restrictive than a paper book.

--edit-- I realise that I first said ebooks and now am referring almost exclusively to amazon. It's entirely possible that other schemes could allow transfer of ownership relatively easily. I'm not aware of any that do right now, but there you are.

Books can be shared, traded, or resold.

A license is a contract one side drafted and the other side did not read.

. . .or read and assumed, rightly or wrongly, that they could live with the limitations of the license.