Dash Commerce is pretty much the default open source c#/.net e-commerce platform. It's got the biggest community, is under active development, has Enterprise options (ie, if you want to, you can pay for support) and is actually used by a lot of websites, so you can be fairly confident it's mature, reliable etc. There are also lots of payment plugins already available, which is a bonus as payments are tricky and need to be done securely and reliably. Unfortunately Dash Commerce seems to have disappeared off the face of the internet. nopCommerce is the next most mature and active ASP.Net Webforms alternative.
IIRC it was originally written by Rob Connery as a project to use Subsonic, but it's matured and grown it's own self sustaining ecosystem since then.
Rob has also created the MVC Storefront as an ASP.Net MVC learning exercise. It's probably less mature and harder to use than dash commerce, but if part of the exercise is the technical challenge, then it might be very interesting.
