If ebooks were sold on ROM chips that could be read in an SD slot, you could pretty well sell them the same way as pbooks. Imagine this, you choose your titles and the book shop attendant burns them to a chip right there before your eyes. Or a simple vending device connected to internet provides the same functions. I would be willing to buy my books that way. Of course that would set fire to internet shopping.
Some other related ideas were discussed
here.