Hey again,
If you are wanting to base it on a CMS and if .NET is a requirement then I would go with SiteFinity and then customise it. DotNetNuke has been around longer, but SF is our current CMS of choice and we are actually wanting to use it for an upcoming eCommerce project.
Depending on the complexity of your eCommerce requirements, you can go about creating your shopping carts, payment processing, back end functionality (catalogue, order etc management) yourself, but I would also have a look at...
http://www.aspdotnetstorefront.com/
Thanks,
-Pat Ramadass
The only source of knowledge is experience.