\nFor the purposes of this subchapter, proof of the existence of a weight or 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 shall be presumptive proof of the regular use of such weight or 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.
Pennsylvania Legal Code