Any contract entered into or carried out in whole or in part, in violation of the provisions of section 73.6, shall be void and the contract or any claim growing out of the sale, delivery, or use of the coal specified in the contract, shall be unenforceable in any court. In addition to any other proper party or parties, any unsuccessful bidder at a letting provided for in section 73.6 shall have the right to maintain an action in equity to prevent the violation of the terms of section 73.6.