As an effective means of price discrimination, some suppliers offer credit periods to the buyers in order to stimulate the demand for the product they produce. The availability of the credit period from the supplier enables the buyer to choose his selling price from a wider range of price option. Since the buyer's lot-size is affected by the customer's demand rate of the product, the problems of determining the selling price and the lot-size are interdependent and must be solved simultaneously. In this regard, this paper deals with the problem of determining the buyer’s optimal selling price and lot-size simultaneously when the supplier permits delay in payments for an order of a product. The positive effects of credit period on the product demand can be integrated into the inventory model through the consideration of retailing situations, where the demand rate is a function of the selling price the buyer sets for the product. Thus, it is also assumed that the customer’s demand rate is represented by a downward slopping linear function of the selling price. Investigation of the properties of an optimal solution allows us to develop an algorithm whose validity is illustrated using an example problem.