793. The taxpayer’s income attributable to business done with members is computed, for the purposes of section 792, by establishing the portion of the taxpayer’s income, before any deduction permitted by this Title, that the proportion that the value of the business done in the year with the members is to that made with all customers. Such business includes the value of the goods or products which the taxpayer has acquired during the year from his customers or has marketed on behalf of his customers or has sold to them or the services which he has rendered to them.