You're right of retaining rights-- I was thinking of vanity publishers and not epubs-- an epub would likely buy your rights and not allow you to keep them.
As far as Ellora's cave, Samhain, et all, these are stocked in stores becuase they are also print publishers. I was thinking of "epub" in the strictest sense of the word. Ellora's cave has had unparallelled success in the epublishing world (Though one can argue that a couple of others--samhain being one of them-- are close on its heels).
And yes, SOME books sell thousands of copies and go on to be traditionally published but those are one of thousands of others that do not, and I think if someone is getting into epubbing with this being their sole goal they will be disapointed. They'd have better luck getting an agent and going the traditional route. Epubs, in my opinion should NOT be looked at as a direct route to new york, but rather as their on sub-industry.