I was cleaning up an ePub doc I created with Sigil and I came across a web based ePub validation site. It is free to use and provides some valuable info about the ePub doc.
http://threepress.org/tools/
I also found a site that will publish an original ePub book on the iBook store for use with the iPad. As long as you have a validated ePub file, they will assign an ISBN number. Apple gets 30% of the ePub price, the company gets 20% of the remaining 70% and the author gets 80% of the 70%.
http://www.lulu.com/apple-ipad-publishing