Bravo, but you shouldn’t have bought one with a stubby in first place
Presumably you mean you can’t unscrew it. If you apply too much force, the screw at the base will shear and you will have to undo it from the inside. So you might as well undo it from the inside (get your head in the boot with a light and look up under where the aerial is), then you still have an intact unit to sell. It’s a little fiddly but not too difficult.