Quote - With Millions of Users ...

It seems reasonable to think that their might be a market for art, artwork on physical items (T-shirts, mugs etc) and possibly for "tube" like products.
I can't see 3D products getting a big boost in sales, as the number of people that have FB accounts but not accounts at one or more of the current stores seems likely to be tiny (as compared to vice versa).

Quote - I think they might just be the new storefront of choice for merchants.

It could well be an additional venue for merchants, but for various reasons (enumerated below) I doubt it would be popular with customers.

Would you really trust FB with your CC info ?
It's almost certain that it would be as awkward to keep what you've bought private, as most data people expose to FB is.
It would be a copyright/trademark nightmare.
There wouldn't be any of the testing and QC as done by existing 3D stores.
FB would likely take a larger cut than existing stores to support the staff needed to handle all the back office functions (shipping, download resets, refunds, chargebacks, disputes, copyright and other complaints etc.)