The act of selling ebooks is a service, the books themselves whether it is a paper or digital version is a good.
Any digital good that is
not locked with DRM
DOES fall within First Sale doctrine. There was a lawsuit from 2007-08 about CDs (which is also digital media on a physical medium) being sold second hand and the company UMG did not like it.
Example lawsuit:
http://arstechnica.com/tech-policy/n...tional-cds.ars
So it doesn't matter if it is a physical book, ebook or Kindle with books on it, the law states we can legally sell the digital goods as we please provided it does not have DRM from the source we purchased it from.