This is a late reply.
I have used a multitude of shopping carts. Even if not for the extensibility of Joomla, Virtuemart has proven far better for its ease of use and customization. While it does have some quarks, such as limited resources when it comes to payment gateways and shipping modules. Nevertheless, I run several large volume shopping carts with it.
If you're using K2 and expect to also make use of Virtuemart, you'll have to solve a particularly daunting conflict issue with the Ajax in the category search field on the backend product management utility. Otherwise, like me, you will have to drop any hopes in using K2, since there are conflicts that have yet to be addressed in the latest versions of K2, and I doubt they will be addressed in future versions, since I have not had much feedback on the subject, with the exception of some Twitter coorespondence from Fotis several months back.
I have several forum posts regarding this issue but only managed to find one.
k2community.joomlaworks.gr/forum/topics/k2-conflict-w-virtuemart-ajax 
