There are many ways for an author to market their eBooks online.
You could create an author blog to promote your books and yourself, and offer a free sample (maybe 2-3 chapters) to people who follow you or register on your site. You can also build up a mailing list by using this method.
Give a free copy of your ebook to websites/blogs that review books. If your book is reviewed on their sites, you will get broad exposure and reach a much larger audience than you are able to on your own site or blog.
Use social networks such as Facebook and Twitter to let followers and friends know that you have published an eBook, and include a link/s to the site/s where they can purchase your book. Also, ask friends and family to review your book online, on the site where it is sold, such as on Amazon or iBooks.
Hope this helps...