There is a paypal-authorized program called
Linklok that I've used on my site. (The site is presently down for restructuring, but when it is back up, I'll be using it again.)
You have to be able to work with some basic cgi scripts, mostly just entering data where it tells you, and storing that in your cgi-bin. The script allows you to set up sales on your site that click through to Paypal, accept payment in CC or Paypal account form. Once paid, Paypal authorizes the software to send an email to the recipient, with an encrypted link to the book on your server. (The link is encrypted... not the ebook.) You can set the parameters of cost, location of the download link, and how long it stays active for the user.
I recommend it for ease of use and effectiveness, and it's only $30.