This post speculates on how ebooks will eventually affect the used books market in the US:
http://walkitout.livejournal.com/1055353.html
The writer suggests that the supply of used books has exploded as a result of ebooks, and also because the internet makes distribution easier. But, this may eventually result in a bust, because if enough people switch over to ebooks, they may not be interested in buying physical books anymore, and for the businesses, an increase in supply means a fall in prices.
I think that as publishers continue to digitize their back catalogs (and in the very long term, as more books fall into the public domain), it may be just cheaper to buy an ebook, because you don't have to pay for shipping. And that will hurt online used book sales.