An eBook is delivered to your e-reading device instantly.
The International Standard Book Number, or ISBN, is the number associated with the scan bar you see on every modern book today, and you must have one if you want to sell your ebook on Amazon as a real publication.
You can submit your ebook to Amazon, who will sell it for you at their site for a large commission, currently 55%, even for ebooks.
Alternatively, you can sell your ebook yourself on eBay or on your own website.
Paypal has tools on its website you can use to set up an automated download of your ebook once payment has gone through.