It's been stated that XF is going to focus on being a quality forum, and allowing developers to write add-ons and plugins. I'm not Kier, but it seems rather unlikely that XF would add a shopping cart feature to XF beyond the rudimentary paid subscription system. I just think it's totally beyond the scope of XenForo to deal with tax laws, shipping calculations, inventory management, order tracking, and all the other cruft that a shopping cart requires.
If you are running a sales site that needs forum support, I'd see if you can get someone to modify your shopping cart software to automatically create an account in your XenForo forum, populate the user account with the relevant information, and then e-mail the user information on how to login to and use the forum should they be interested in using it. Most purchasers of products don't feel the need to comment on it unless there's a problem.