In determining, pursuant to the provisions of this section, what markets are bona fide spot markets, the Secretary is directed to consider only markets in which spot cotton is sold in such volume and under such conditions as customarily to reflect accurately the value of middling cotton and the differences between the prices or values of middling cotton and of other grades of cotton for which standards shall have been established by the Secretary; except that if there are not sufficient places, in the markets of which are made bona fide sales of spot cotton of grades for which standards are established by the Secretary, to enable him to designate at least five spot markets in accordance with subsection (f)(3), he shall, from data as to spot sales collected by him, make rules and regulations for determining the actual commercial differences in the value of spot cotton of the grades established by him as reflected by bona fide sales of spot cotton, of the same or different grades, in the market selected and designated by him, from time to time, for that purpose, and in that event differences in value of cotton of various grades involved in contracts made pursuant to subsection (f)(1) and (2) shall be determined in compliance with such rules and regulations. It shall be the duty of any person engaged in the business of dealing in cotton, when requested by the Secretary or any agent acting under his instructions, to answer correctly to the best of his knowledge, under oath or otherwise, all questions touching his knowledge of the number of bales, the classification, the price or bona fide price offered, and other terms of purchase or sale, of any cotton involved in any transaction participated in by him, or to produce all books, letters, papers, or documents in his possession or under his control relating to such matter. A person complying with the preceding sentence shall not be liable for any loss or damage arising or resulting from such compliance.