The board may itself make any investment authorized by law or sell any security, obligation, or real property in which moneys in the retirement fund are invested, by affirmative vote of at least seven members of the board, or by the same affirmative vote may from time to time adopt an investment resolution that shall contain detailed guidelines by which to designate those securities and real property that are acceptable for purchase. While the resolution is in effect, securities and real property may be purchased for investment by an officer or employee of the board designated by it for that purpose, and sales of securities may be consummated by the officer or employee under the conditions prescribed. Purchases and sales of securities shall be reported to the board, on a monthly basis, at its next regular meeting.