I was working on a ecommerce site for reliant-rehab when I first get to know nopCommerce, its a true open source ecommerce site, build using asp.net. Pretty handy and easy to customizable, I am not going to explain each and every detail, the installations steps are pretty simple and you can download the from bellow site.
You can customize the theme and controls and bend it as you like it. There is a documentation for nopCommerce and users can buy it paying a small about of money 20$. This part is pretty strange, if its true open source ecommerce site the documentation should be also free . Any way they have there own policy, but as developers if we explore a little bit its easy to customize the site. Bellow I have added a screen shot for one of my work.
Start building awesome web applications, best of luck happy programming.