Thaler (1985) proposed general principles of mental accounting regarding the phenomenon whereby the realization of losses and gains is differentiated according to prospect theory's asymmetric value function. Gains are realized separately while losses are realized in an integrated manner; small losses are realized by integrating them with relatively large gains, whereas small gains and relatively large losses are realized separately. Investigating whether such structural bias phenomena—whereby individual investors' stock selling behavior is differentiated between gain stocks and loss stocks according to mental accounting—also appear in the stock selling process of individual investors in the Korean stock market carries significant implications. If the asymmetric selling propensity driven by loss aversion in prospect theory is the main effect observed in individual investors' stock selling decision-making process, then the phenomenon of selling holdings separately or in an integrated manner based on gains and losses constitutes an interaction effect according to mental accounting. Empirical research analyzing such interaction effects using actual transaction data represents an important step toward diversifying the process of explaining individual investors' selling behavior through prospect theory. The focus of this study is an empirical analysis of the cross-sectional differentiation phenomenon in the scope of selling based on gains and losses, going beyond the asymmetric selling behavior (disposition effect)—the time-series differentiation of selling timing for loss stocks versus gain stocks—in individual investors' selling decisions. The analysis of actual trading data from individual investors who traded through a mid-sized securities firm generally supports the general principles of mental accounting proposed by Thaler (1985).