If you're serious about developing and publishing ebooks, my advice would be to not distribute them as PDFs - or, at least, not exclusively. PDFs are designed to show fixed page sizes, and these don't work well with hand-held ebook readers like the Kindle, Nook, or smartphones.
Much better to do it in Kindle and ePub formats, which support flowable pages. With these formats, the book will fit well with any device, and can also be read on full-size laptop or desktop PC screens.
If you opt for Kindle format, you will be able to distribute your book through the Amazon publishing platform, which will remove all the hard work of physical distribution, billing and collecting payment (but at the cost of giving Amazon a cut in your earnings). Similarly with Barnes & Noble's PubIt system for ebooks in ePub format. But, either way, you will still have to promote the book yourself, which you would do via your own website, with a "Buy now" link to Amazon or B&N.