It's not setup as a plugin, but as a theme.

If you want to integrate the shopping cart functionality into your own existing theme, the license does give you permission to do that, but you'll have to modify & add some of the code of your theme for it to work.  (or hire someone to do it for you)

The backend product administration portion of the system is a plugin so that can be installed without regard to which theme you're using, but the shopping cart has a bunch of code within the theme.

In a few months, we're looking into getting some theme variations made of the latest Market version.  That should give people a larger selection to draw from when implementing.