The provisions of subsection (b) of this section shall not apply to any purchase and sale, or sale and purchase, and the provisions of subsection (c) of this section shall not apply to any sale, of an equity security not then or theretofore held by him in an investment account, by a dealer in the ordinary course of his business and incident to the establishment or maintenance by him of a primary or secondary market (otherwise than on a national securities exchange or an exchange exempted from registration under section
78e of this title) for such security. The Commission may, by such rules and regulations as it deems necessary or appropriate in the public interest, define and prescribe terms and conditions with respect to securities held in an investment account and transactions made in the ordinary course of business and incident to the establishment or maintenance of a primary or secondary market.