For the purposes of this chapter, proof of the existence of a weighing or measuring instrument or device in or about any building, enclosure, stand, or vehicle in which or from which it is shown that buying or selling is commonly carried on, shall, in the absence of conclusive evidence to the contrary, be presumptive proof of the regular use of such weighing or measuring instrument or device for commercial purposes and of such use by the person in charge of such building, enclosure, stand or vehicle.