No teacher shall act, directly or indirectly, as agent of, or be financially interested in, any commercial venture which furnish textbooks and other school commodities in the purchase and disposal of which he can exercise official influence, except only when his assignment is inherently, related to such purchase and disposal; provided they shall be in accordance with the existing regulations; provided, further, that members of duly recognized teachers cooperatives may participate in the distribution and sale of such commodities.