Va. Code Ann. § 3.2-5648

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