Unfortunately a real product license and trademark costs money and unless you are paying that fee and registering it with the government, The supposed license that thingerverse is giving you is not abiding and won't hold up in court. That's just the way of it. So unless you are willing to go to the trademark commission to file a trademark, or the patent office to patent the stl file. You're all shit out of luck. And then the seller will just switch and say he is not selling your items he's selling his time and the filiment used to make the item. Stop posting to thingiverse if you don't want your items used it's that simple.