FORMAT-STORE is a serious game application designed for training salesmen and managers in the context of a retail store or a larger supermarket. In this paper, we argue that a relevant way to train a salesperson to their daily activities (e.g. customer relationship management, store management and stock control) consists in immersing them in a 3d environment populated with realistic virtual customers. The first part of this paper presents the multi-agent approach we apply to the design of the intelligent customers. Specifically, we analyse the contribution of the interaction-oriented methodology I ODA in facilitating the conception of a game for non computer-scientists by means of a user-friendly design tool and the automated implementation of the conceptual model. The second part includes a case study where we present the organisation of the game around scenarios modelled with respect to the pedagogical requirements. We discuss how we intend to engage the learner by carefully adjusting the difficulty of these scenarios.