NopCommerce

As a web developer I love open source solutions.
nopCommerce is one of those open source solutions which can help you to build powerful highly customized bespoke online stores.
Because this utilizes C# MVC you can do just about anything with it.

I find it to be a very secure platform which is really good to work with.
building plugins is not too difficult, in fact once you have built your first one it's rather simple.
There are a bunch of plugins available as visual studio solution projects, so you can analyse their code to see how things are done when developing your own customizations and the same is true for nopCommerce itself. The nopCommerce have put together a very well coded solution, and because all the code is readily available to you uncompiled you can use it as a base for further development.