If the business is one to which the retail concept is applicable then the second requirement for qualifying as a “retail or service establishment” within that term's statutory definition is that 75 percent of the establishment's annual dollar volume must be derived from sales of goods or services (or of both) which are recognized as retail sales or services in the particular industry. Under the Act, this requirement is distinct from the requirement that 75 percent of annual dollar volume be from sales of goods or services “not for resale” (§ 779.329); many sales which are not for resale lack a retail concept and the fact that a sale is not for resale cannot establish that it is recognized as retail in a particular industry. (See Wirtz v. Steepleton General Tire Co., 383 U.S. 190.) To determine whether the sales or services of an establishment are recognized as retail sales or services in the particular industry, we must inquire into what is meant by the terms “recognized” and “in the particular industry,” and into the functions of the Secretary and the courts in determining whether the sales are recognized as retail in the industry.