@david_james6, simply go to page of product that you want to buy and on the right side look for tab "licence". In it you can see under what licence product is selling. You can click on licence link there and pop up window will appear with text of that licence. If we speaking about general licence, then you can see that it specifically allows you to 3d print physical object and to do with it whatever you want (2.2(4)). That 3d printed physical object is your property, unlike 3d model file that you bought.
Iguess, people who said you can't sell 3d printed object, simply don't know about licence agreement under which they sell models. They just upload files and hope that seller will comply to whatever imaginery conditions they have in their minds :]