One of the challenges of selling ebooks (pre-Kindle) has been the plethora of formats available.
This strikes me as both a strength and a weakness of ebook sellers. If you've got any kind of device with a screen you can probably find an ebook reader that supports one of these formats. But as a new user hitting the site, the choices can seem overwhelming. Presumably this is the reason for the more focused eReader.com site: Step 1, download our reader software for your device. Step 2, Start buying books from us.
This is also why Amazon probably has the best shot of taking ebooks mainstream.
Barnes & Noble abandoned ebooks once, so why are they coming back to them now? Because the format is starting to take off. Why is that? What's popular on Fictionwise? Well, once again it seems like porn is blazing a path to a new media format. Of the top 10 bestsellers under the "Multiformat" category, nine are tagged "erotica" amd the last is "dark fantasy".