2 U.S.C. § 150

The Librarian of Congress is authorized to furnish to such institutions or individuals as may desire to buy them, such copies of the card indexes and other publications of the Library as may not be required for its ordinary transactions, and charge for the same a price which will cover their cost and ten per centum added, and all moneys received by him shall be deposited in the Treasury and shall be credited to the appropriation for necessary expenses for the preparation and distribution of catalog cards and other publications of the Library.