This is not exactly gonna help you sell ebooks, but i came across this interesting free back office "system" for ebooks:
www.scrobbld.com
If you use Paypal, scrobbld can help you keep track on sales/inventory and customers, and it will also send an email to customers when they pay for your book. I use scrobbld, even if i dont sell ebooks, its anyway a good back office system.
Its free as long as you have 3-4 sales a day.