Amazon's Kindle eReaders are some of the most popular ways to read ebooks, and although they are connected directly to Amazon's online marketplace for purchases that doesn't mean that your buyers won't be able to read your ebooks on their Kindles, even if you're not selling on Amazon's site!
It's very easy to import a variety files to a Kindle including MOBI, Word documents or PDFs. Each kindle device has a unique email address assigned to it, so you attach files to an email message and send them straight to your Kindle for access. To find the address for your Kindle device and applications, check the Manage Your Devices page at Manage My Kindle.
You will also need to add your own email address to the list of approved addresses that can contact your Kindle, by visiting the Personal Document Settings at Manage My Kindle.
PDF files can be sent as is or converted to the Kindle format to use its extra features if the subject line of the email message is "Convert".
Of course, it's still possible to connect the Kindle to another device and transfer files from there. If you plug your Kindle into a desktop computer via USB you can copy or drag files from your hard drive into the Documents folder on your Kindle. This is particularly helpful if you don't have a wireless connection to use otherwise. The specific location of the Documents folder within your Kindle will vary depending on which specific device you have, this help page covers how to find which device you have and provides the specific instructions for accessing the Documents folder.
Amazon also offers applications for desktop computers and even Android phones that allow you to quickly transfer files to a Kindle for easy access.
If you have a Windows PC the application will allow you to send files to your Kindle by right clicking them from your desktop/Windows Explorer: Send to Kindle for PC.
For Mac users, the application will put a Kindle icon on your dock where you can drag and drop files, or you can Control+Click to send them over: Send to Kindle for Mac.
And lastly, for many applications for Android devices will contain a Send to Kindle button as long as you have also installed the Kindle app for Android: Send to Kindle for Android.
So even if you aren't selling on Amazon.com your ebook can still easily be read via Kindle, and many Kindle owners will already know how to take advantage of these features. For those who don't know and may ask before or after purchasing check the links above to find the relevant information they might need. Happy selling!