Selling a book online can be a mysterious task for those authors who have not done it before. Many still wish to hand the manuscript over to a publisher and have them do all the legwork for them. That's great but you will pay for the privilege forever as they take their large portion of the proceeds regardless of the fact that they have absolutely no additional costs between book number 2 and book number 2,000.
I'm currently working with a client who has written a wonderful children's book targeted at military families (I'll put up a link when we have everything ready). She is launching the book soon with a first edition of hard copies to a local audience and has asked me to make it available online. I have done this with other books and so, I thought that I should make you aware of some of the changes and differences that you should expect if you decide to do the same.
When you create a hard copy book, you can use any of thousands of font styles, include illustrations in any way you like, and can format your pages just exactly the way you want. This is not the case with e-books. I think this is changing as the devices and software change and evolve but at the moment, there are some restrictions and the best way to understand these is to use an e-reader yourself. For those of you who create books that are reliant upon design and formatting rather than plain text, this will be a challenge. Children's books are a good example of needing to alter your expectations for e-readers. Once again, I am pretty sure that will change drastically as you will be able to embed videos, audio and everything else into your books but for now...for the conventional e-reader, here's some differences:
Fonts: probably best to stick with one font, including chapter titles and headings. Preferable to stay with something like, Times - New Roman, for the time being but you can use a couple of different sizes ( but make the normal text size 12 point) and bold the chapter headings. The eventual reader can change both the font style and the font size on their machine, which is why you need to be consistent in your source document.
Illustrations: At this time, most conventional e-readers are black and white based, but this is changing rapidly so color should be OK. Illustrations don't load particularly easily yet but this is getting better all the time. Quite often the e-reader screen size is rather limiting and can make clarity and readability of the pictures, maps and so on, difficult. Once again, devices like the I-Pad and other tablets have this in control and are much better than the conventional e-reader.
Right-hand justification: forget it! Because the reader controls sizing, right hand justification simply does not translate for e-readers. You still might like it for hard copy books but your source document for electronic books needs to have it removed. The same is true for things like centering and a lot of other formatting - best to keep it as clean as possible.
Page numbers: this process has improved immensely. Initially, e-readers couldn't handle page numbers and used a combination of percentage of book read and some convoluted system of determining book locations. Now, you can use page numbers and the various upload software that is available will guide you through the actual handling for each one.
Table of Contents: don't miss the opportunity to provide an active table of contents for non-fiction books. Most are not read "front to back" and the reader wants to read them out of order or maybe some parts, not at all. I'm thinking cookbooks, travel books and the like. You will normally add the "active table of contents " formatting in your source document - Word, for instance, has a process available for this and it can be found in their help section. Frankly, if you don't include that in most non-fiction books that have many chapters and sub-chapters/headings, you will succeed in ticking off your audience, royally.
ISBN's: I just think that it is best to acquire your own ISBN's from Bowker, or from the source in your own country, however, that is no longer always true. In some instances, you can obtain a free one through CreateSpace, for instance, or Smashwords that will then give you better access to their expanded distribution channels. Read the instructions and guidelines provided by the various outlets carefully BEFORE you make a decision that you cannot reverse. Do what's best, not necessarily most expedient,  for you as the author. Remember that you will need a new ISBN for each book format that you create.
Design: For most fiction writers who create straight forward text based books, the whole process is reasonably simple but for those people who imagine the possibility of adding additional features to their book that will enhance the reader's enjoyment ( and perhaps their sales), it might be an idea to hook up with a good designer. When I say designer, I am thinking of someone who has an excellent understanding of new media and how to create it. Someone who can create graphics, video, photographs and sound that can be embedded in a document for publishing to the Apple, Kindle, Nook,  Android and other stores or retail outlets. The non-fiction market is about to explode as these kinds of books become available. Imagine writing a house renovation book that shows in color and graphic detail exactly how to instal that new hardwood flooring with a combination of sound, video and print instructions. Imagine what this technology is going to do to the textbook market and how it will revolutionize education.
Selling Your Book Online: A few final thoughts about what's possible for you in the future. As the formats and technology change, your opportunity to create a revised edition for your book changes, too. Unlike the traditional bookstore, an electronic bookstore can accept and list an unlimited number of titles so that you can update the text, create videos ( how-to's, travel, cooking, etc.), release an audio version and lots of other things to publish a new edition of your book and still keep the original available for those people who continue to use the older style e-readers. Your costs don't substantially change and you don't need to pay for additional printing, shipping, etc.
If you have added some of this exciting new technology to any of your books, or are thinking about it, please leave a comment and a link so that we can all see how you are doing.