I use leanpub and gumroad for my technical books. Both of them are non-exclusive, you retain the rights and ability to sell anywhere else you like. Leanpub has

I use leanpub and gumroad for my technical books. Both of them are non-exclusive, you retain the rights and ability to sell anywhere else you like. Leanpub has 20% cut, but that's probably because they allow you to generate the ebook in pdf/epub/mobi formats from their website. An important feature I like is the ability to update the books (due to s/w versions, typos, adding new content, etc) and notify the users regarding the new version. Another feature for both sites is that customer can pay more than the price of the product. And that does happen often enough to make it a highlight of using their site. But this may probably apply more for technical books than fiction.