customized ecommerce

I want to build a seo optimized complete ecommerce website. Along with the regular features it should have some ebay features such as multiple quantity, multiple prices and fixed time duration on each sale. The project should preferably be php/mysql and modular so that I can easily scale and add new functionality such as external sellers similar to amazon.com. The admin should be easy to operate and handle product catalogs, inventory, shipping, billing, state taxes.