This paper argues that the advancement of large-scale language models (LLMs) is transforming AI agents from passive computational tools to autonomous economic actors, and that this transformation signals the emergence of an agent-centric economy. Accordingly, we propose Agent Exchange (AEX), a specialized auction platform designed to support the dynamics of the AI agent market. AEX provides an optimized infrastructure for agent coordination and economic participation, and is inspired by real-time bidding (RTB) systems in online advertising. AEX acts as a central auction engine that facilitates the interaction between four ecosystem components: the User-Side Platform (USP), the Agent-Side Platform (ASP), the Agent Hub, and the Data Management Platform (DMP). In this paper, we outline the design principles and system architecture of AEX, laying the foundation for an agent-centric economic infrastructure in future AI ecosystems.