For a start I would actually try and do something different from eBay initially just to offer USP/differentiation. However, assuming you want to create an eBay

For a start I would actually try and do something different from eBay initially just to offer USP/differentiation. However, assuming you want to create an eBay for use in your own site/society/community and competing with eBay is a non-issue, I would do it in the tools that allow building as easily and quickly as possible. I would use python/django because of simplicity in code style and they are good opensource frameworks for building web apps that incorporate many good paradigms. Others may prefer what is in thier skillset e.g. RoR/.NET/PHP, but that is preference... the auctomatic guys started building in common lisp. My approach would be KISS, working on the core essence i.e. an auctioning engine, and building iteratively there upon. I would also initially start by looking at what is broken in eBay and what people generally complain about and try to incorporate the learning into my roadmap but still focusing on the core competencies first.