I believe the creative commons licence that most 'free' models have allows for the sale of the printed item.
Just not commercilisation of the stl file.

You could also argue that once you've finished the model and painted it, the value is in the painting and finishing not the original unpainted figure.

If you are buying the stl's - then that's a different set of licences entirely.

But for the free models - there should not be any legal issues in you selling the physical models you print.