No person employed under the authority of the United States, in the collection of duties on imports or tonnage, shall own, either in whole or in part, any vessel (other than a yacht or other pleasure boat), or act as agent, attorney, or consignee for the owner or owners of any vessel, or of any cargo or lading on board the same; nor shall any such person import, or be concerned directly or indirectly in the importation of, any merchandise for sale into the United States. Every person who violates this section shall be liable to a penalty of $500.