If you're using Wordpress there is a plugin specifically for this called WordPress eStore that I've used. It's surprisingly painless, works with the first try for the most part, and has pretty good support turnaround on the site. I've temporarily suspended it's use on my site while I focus on marketing for my Amazon and Smashwords links, but I'll bring it back soon I expect.
http://www.tipsandtricks-hq.com/word...-securely-1059
Tenhawk AKA Evan Currie
Author, Thermals
http://www.amazon.com/dp/B004URUXKQ?tag=fiwi0f-20
http://www.smashwords.com/books/view/53324
Follow me on Twitter at
http://twitter.com/#!/tenhawk
www.tenhawkpresents.com