It depends on the type of book. For fiction this is definitely true. For textbooks and technical books, there is a massive used market with high resale prices, which is more what I was thinking of. I suspect a lot of these vendors simply won't jump on the bandwagon without some kind of reassurance.
My guess is that some people will do it accidentally, but that the number of people doing it as a book cost-reduction strategy will be pretty low. The net yield on selling used books is not good. And I'd guess further that for 98% of on-sold physical books, people will never go back and look at the ebook again.