For the purposes of this act, proof of the existence of a weight, measure or a weighing or measuring 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, in the absence of conclusive evidence to the contrary, shall be presumptive proof of the regular use of such weight, measure or weighing or measuring device for commercial purposes and of such use by the person in charge of such building, enclosure, stand or vehicle.