A build-to-order (BTO) manufacturing system is excellent for enabling short delivery time with product variety; however, it is difficult to control inventory, and an unbalance of parts consumption causes the problems of stock-out or surplus parts. This study focuses on a stock reduction method for BTO products to flow surplus parts out to the market using sale by recommendation. The method is based on the potential of a customer specification to be replaced by an alternative one if the alternative one is close to the initial customer specification. A recommendation method is proposed that decides the recommended product configuration by balancing the part consumption so that the alternative specification of the configuration is close enough to the initial customer specification for substitutability. The method was evaluated by a simulation using real BTO manufacturing data and the result demonstrates that the unbalance of the consumption of parts inventory is improved.