An e-retailer collaborates with other e-retailers to satisfy customer requirements in various ways. For example, in affiliate marketing the affiliate (or associate) e-retailer promotes the products and services of the merchant e-retailer on the affiliate’s website for a commission. Another collaboration type is the cooperative exchanging mechanism proposed by Ito et al. in which an e-retailer can online exchange for a product if he does not have enough of the product for sale, with the aid of software agents. In this study, an online collaborative stock management mechanism for multiple e-retailers is proposed, in which a product can be simultaneously for sale on multiple websites, and the stocks of the product on these websites are monitored, virtually gathered together, and shared among websites.