In the fine print that you "agree" to, Amazon and Sony say you just get a license to the e-books—you're not paying to own 'em, in spite of the use of the term "buy." Digital retailers say that the first sale doctrine—which would let you hawk your old Harry Potter hardcovers on eBay—no longer applies. Your license to read the book is unlimited, though—so even if Amazon or Sony changed technologies, dropped the biz or just got mad at you, they legally couldn't take away your purchases. Still, it's a license you can't sell.