No producer, manufacturer, or dealer shall ship or deliver for shipment in commerce any goods produced in an establishment situated in the United States in or about which within 30 days prior to the removal of such goods therefrom any oppressive child labor has been employed: Provided, That any such shipment or delivery for shipment of such goods by a purchaser who acquired them in good faith in reliance on written assurance from the producer, manufacturer, or dealer that the goods were produced in compliance with the requirements of this section, and who acquired such goods for value without notice of any such violation, shall not be deemed prohibited by this subsection and conviction of a defendant for the shipment or delivery for shipment of any goods under the conditions herein prohibited shall be a bar to any further prosecution against the same defendant for shipments or deliveries for shipment of any such goods before the beginning of said prosecution.