Among features often attributed to software agents are autonomy and mobility. Autonomy of e-commerce agents involves adaptability to engage in negotiations governed by mechanisms not known in advance, while their mobility entails such negotiations taking place at remote locations. This paper aims at combining adaptability with mobility, by joining rule-based mechanism representation with modular agent design, and at UML-formalizing selected aspects of the resulting system. Furthermore, we discuss the issue of agent mobility and argue why such agents have been proposed for the system under consideration.