Hi Dan, I am also looking into doing this. There are several apps to serve as a ecommerce sitewhich I have taken a look at but not really look and do the things I would like. The way I am going to approach this is by building my own site for the shop, im not a programmer so have looked at software to use to help do it. The one I have opted for is by Serif Web Plus x5 which has lots of tools to build the site, im also going to use it to build other pages as I can then have lots of the functions available, ie toolbars,drop down lists,filters and even its own forum, etc. Once the shop is built all I will do is iframe into a page on my site. Im going to set up the template to fit the full page option on my ning site so it fits perfectly, it should work really well as I have iframed other members website in a page and it functions as it should. The beauty about creating the site offsite is its easy to make changes without interrupting your ning site and also if your have your site closed so only non members see the main page if you create an advertising page this can always be available for non members to click on so it is a plus for any potential advertisers signing up with you.
Hope that helps and gives you some ideas.