This study seeks to present a new perspective on the trade-in sales phenomenon that has recently gained significant prominence in business practice. The prevailing practical understanding of trade-in sales is that they serve as a promotional tool utilizing price discounts. In this study, trade-in sales are viewed as a form of third-degree price discrimination serving as a price differentiation mechanism, and based on this perspective, a theoretical explanatory model of trade-in sales as price differentiation is presented. Three implementation conditions related to the execution of trade-in sales as price differentiation are proposed. Furthermore, this study analyzes "AVc" (Additional Value from disposal of existing products) as a critical strategic variable that influences the optimal trade-in price in the implementation of trade-in sales. The potential utilization of trade-in sales and AVc as a means of securing competitive advantage is also examined. In particular, trade-in sales as a price differentiation strategy under competition are analyzed using Cournot and Stackelberg models in a duopoly setting, demonstrating that AVc is a crucial strategic variable for firms in implementing trade-in sales. This study represents the first approach in marketing to the trade-in sales phenomenon, and its significance lies in developing a theoretical model based on the novel perspective of price differentiation and providing normative criteria for the utilization of trade-in sales as a marketing strategic tool by analyzing price differentiation through trade-in sales under competitive conditions.