A few years back, I enlisted the help of an e-book agent that specialized in e-book sales. Part of the package was to make my e-book available on about 20 or so platforms. But I couldn't get a straight answer from these people, so I suspected a cascade of bills was their business plan, seeing how much money they could milk out of me. Best not to start.
I opted to do a more limited coverage on my own: Amazon, Kobo, and iTunes. I was able to set up an e-book on the first two, but iTunes required "more technology". So I dropped that platform.
From my studies, Amazon has about 50% of all e-book sales. Sales have not been great, so I doubt finding another e-book reseller--with a business model more to my liking--would generate more sales. But maybe I should send some emails--and see what happens.
And my four books are free from my website anyways. I consider my alternative democracy too important to let a small fee or technology be a barrier.