Among the existing E-Commerce applications, online auctions are the most influential ones. Their impact on trading in the B2B (business to business) as well as in the B2C (business to consumer) and C2C (consumer to consumer) areas will be inevitable. This article describes the architecture of a web-based realtime online auction system, together with the functional and technical requirements that evolved during the development process and heavily influenced the architecture. From the point of view of this real world case study, ways to minimize the development time and yet ensure a robust and flexible system are explained: combining standard software and self- developed components, reusing code wherever possible, and employing the eXtreme Programming approach and its test concepts.