§ 21. Commissions; penalty. An auctioneer in any county, other than New York or Kings, shall not, without a previous agreement in writing, with the owner or consignee of the goods sold, demand or receive a greater compensation for his services than a commission of two and one-half per centum on the amount of any sale, public or private, made by him. For a violation of this section he shall refund the moneys illegally received and forfeit two hundred and fifty dollars to each person from whom he demands or receives an unlawful compensation or commission.