You can sell directly on many popular Internet vendors, which also serve as publishing platforms. You upload your electronic file and they convert it into an eBook. You then include information about the book and choose a sales price. If you choose not to go through one of these platforms, then you'll need to create your own eCommerce site and fight for visibility. The most popular platforms include the following:  Amazon Kindle Direct Publishing Apple iBooks Barnes and Noble Nook Press Google Play Kobo Writing Life Each has different rules for publishing content. You should look around the sites and find each site's terms of service. Read it thoroughly to see whether the vendor allows you to publish public domain content and what terms/conditions apply.  Kobo, for example, will only give you a 20% royalty for public domain titles.  Apple iBooks and Nook Press have also refused to sell public domain work in the past. Amazon won't allow you to publish a public domain title if there is already a free version in their store. You should search the Amazon site to see if a title already is for sale. It's okay if there is a title so long as it is not for sale for free. Amazon, for example, will let you publish the public domain title if you differentiate your book. Your book will be differentiated if you do the following:  Offer a unique translation. This means you translate the book. Don't use an online translation app or use a translation that is in the public domain. Include unique annotations, such as literary critiques, study guides, detailed biographies or historical context. Provide 10 or more unique illustrations that relate to the book. Don't assume that because you found a book on the Internet that it is in the public domain. Also, you shouldn't assume a work isn't copyrighted because it lacks a copyright notice. Instead, you need to individually analyze each book based on the following:  Certain work is not eligible for copyright protection because it is common property, such as calendars or works by the U.S. government.  In the U.S., a book is in the public domain if it was published before 1923. Pre-1923 material is the safest material you can use. If the work was published after 1923 but before 1978, then it is in the public domain if it was published without a valid copyright notice. If the book was published after 1923 but before 1964, it is in the public domain if the copyright was not renewed. You can check if a work was renewed by searching at the Copyright Office. Be careful, however. Works might have been registered under different names. You're inviting a copyright infringement lawsuit if you publish a work that is protected. The copyrights for books published after 1978 will not expire until the middle of this century. The only way it is in the public domain is if the author dedicated it to the public domain. There should be a notice to that effect on the work. Online publishing changes rapidly. Vendors change their terms and conditions at will, and what was legal six months ago might no longer be allowed. Accordingly, stay up to date on publishing requirements.  Join different message boards for indie publishers so that you can keep up on changes to each publisher's terms and conditions. Also check your accounts regularly. Books might be removed from sale without notice, so you always want to check.

Summary:
Identify possible publishing platforms. Obtain publishing requirements from vendors. Check if a free version already exists. Determine how to make your book different. Confirm that the work is in the public domain. Stay informed.