Any retailer who shall, in contravention of the policy of this act, advertise, offer to sell or sell at retail any item of merchandise at less than cost to the retailer as defined in this act, or any wholesaler who shall, in contravention of the policy of this act, advertise, offer to sell or sell at wholesale any item of merchandise at less than cost to the wholesaler as defined in this act, or any person, partnership, corporation or association engaged in business both as a wholesaler and retailer as defined in this act who shall, in contravention of the policy of this act, advertise, offer to sell or sell any item of merchandise at less than cost, subject to both the retail and wholesale mark-ups provided by this act, shall be guilty of a misdemeanor, and, upon conviction thereof, be sentenced to pay a fine of not less than fifty dollars ($50) nor more than two hundred dollars ($200) and costs of prosecution, and, in default thereof, shall undergo imprisonment for not more than thirty (30) days. Each advertisement, offer to sell or sale shall constitute a separate violation of the provisions of this act and shall subject the offender to the penalties hereby prescribed. Proof of any such advertisement, offer to sell or sale by any retailer or wholesaler in contravention of the policy of this act shall be prima facie evidence of a violation of this act.