This rule is no longer applicable. NASD Rule 2440 has been superseded by FINRA Rule 2121. Please consult the appropriate FINRA Rule.
In securities transactions, whether in "listed" or "unlisted" securities, if a member buys for his own account from his customer, or sells for his own account to his customer, he shall buy or sell at a price which is fair, taking into consideration all relevant circumstances, including market conditions with respect to such security at the time of the transaction, the expense involved, and the fact that he is entitled to a profit; and if he acts as agent for his customer in any such transaction, he shall not charge his customer more than a fair commission or service charge, taking into consideration all relevant circumstances, including market conditions with respect to such security at the time of the transaction, the expense of executing the order and the value of any service he may have rendered by reason of his experience in and knowledge of such security and the market therefor.