Not any time soon but you can easily use third party applications for this, unless you plan to allow others to post stuff so like a classifieds? Then its coming some day:

http://www.oxwall.org/roadmap

I do plan on allowing users to post their own downloadable files, but they'd need backend access to track sales, etc. Or, will the upcoming addon do this as well? Looks like it could be a while before we find out. . . 

So, can I add a 3rd party cart, forum AND gallery and somehow bridge them?

Is there any documentation on how to bridge through oxwall???

Nope. Documentation is around the corner for basic functions though, so we are told.

Can you not just utilize the link feature re brand it and call it downloads? Then they can link to a external download rather than it be hosted and downloaded from you as it eats space and bandwidth?

You can add a classified script in another folder on your host than at the bottom of the page or for an advertisement with it linked over to the classified in a new browser. A lot of them allow for Facebook login. Granted it isn't a plugin but it will work. I would really like to have the plugin rather than doing this but oh well. And I am talking about where your contact link and other link are at the bottom of the page. Set it for only members to be able to click over to and that way they don't need to register for the classifieds area. You can check mine out here: http://georgescoggins.com