It is unlawful for any person, firm or corporation to sell or offer for sale at retail, for use or consumption in any motor vehicle, or to deliver into any motor vehicle for actual or apparent use therein, any product for use in supplying, creating or generating motive power to that motor vehicle, or lubricating oil for that motor vehicle, unless the person, firm or corporation conspicuously and plainly posts at the place of the sale or delivery, a sign or placard, stating the price of each such product, and oil, separately, and so that the prices can be readily and easily distinguished by brand or other designation as legible words, letters and figures of uniform size and dimensions. The sign or placard shall be so located and placed that it may easily be seen and read by purchasers or prospective purchasers of the product or oil.