16 C.F.R. § 240.4
A customer is any person who buys for resale directly from the seller, or the seller's agent or broker. In addition, a "customer" is any buyer of the seller's product for resale who purchases from or through a wholesaler or other intermediate reseller. The word "customer" which is used in section 2(d) of the Act includes "purchaser" which is used in section 2(e).
Note: There may be some exceptions to this general definition of "customer." For example, the purchaser of distress merchandise would not be considered a "customer" simply on the basis of such purchase. Similarly, a retailer purchasing solely from other retailers, or making sporadic purchases from the seller or one that does not regularly sell the seller's product, or that is a type of retail outlet not usually selling such products (e.g., a hardware store stocking a few isolated food items) will not be considered a "customer" of the seller unless the seller has been put on notice that such retailer is selling its product.
Example 1: A manufacturer sells to some retailers directly and to others through wholesalers. Retailer A purchases the manufacturer's product from a wholesaler and resells some of it to Retailer B. Retailer A is a customer of the manufacturer. Retailer B is not a customer unless the fact that it purchases the manufacturer's product is known to the manufacturer.
Example 2: A manufacturer sells directly to some independent retailers, to the headquarters of chains and of retailer-owned cooperatives, and to wholesalers. The manufacturer offers promotional services or allowances for promotional activity to be performed at the retail level. With respect to such services and allowances, the direct-buying independent retailers, the headquarters of the chains and retailer-owned cooperatives, and the wholesaler's independent retailer customers are customers of the manufacturer. Individual retail outlets of the chains and the members of the retailer-owned cooperatives are not customers of the manufacturer.
Example 3: A seller offers to pay wholesalers to advertise the seller's product in the wholesalers' order books or in the wholesalers' price lists directed to retailers purchasing from the wholesalers. The wholesalers and retailer-owned cooperative headquarters and headquarters of other bona-fide buying groups are customers. Retailers are not customers for purposes of this promotion.
16 C.F.R. §240.4