You can just add a PayPal button, which is quick and easy. Then he can post or email the book, depending on format. There are tools that can allow the user to download the ebook immediately. These can either mix with PayPal, or you could use a whole range of other services for selling digital goods, like eJunkie or FastSprint. Other carts like FoxyCart mean he can sell both. Many, many, many options for selling online.