No person employed by any public school district shall, either as a principal or as an agent, sell or offer to sell to pupils attending school in the district, or to a parent or guardian of any such pupil, any services or merchandise to be used, or intended to be used, in the schools in connection with activities or studies therein, except under such rules and regulations which shall be adopted by the board of trustees of the district employing such person.
Nothing herein shall limit a board of trustees from purchasing books, supplies or other equipment which may be sold to pupils attending any school in the district.