Hopefully I can save Michael some keystrokes.

1. No, you cannot change a MoI OBJ into a SubD model. There are tools called retopology which can help this, but mostly it's a tedious process.

2. AFAIK, there are no problems sellling on Rendo or other forums a NURBS based topology model vs a SubD one. I used to sell a bunch of stuff on Cornucopia (another Rendo like marketplace) and none of it was SubD. In fact, I would wager most of the content on Rendo is NOT SubD.

Vitaly Bulgarov sells a bunch of SubD and non-SubD models at his kitbash store:
http://vitalybulgarov.com/3d-kitbash/

Some of the models were created in MoI:
http://vitalybulgarov.com/3d-kitbash/black-widow-attachment-blocks-pack

Here's a wire for the same set and is a good example of the mesh options density which works well in poly renderers (click to enlarge):