Reverse auction site

An established business is interested in creating a reverse auction web site in the B2B field. The intention is that buyers can post their demands and receive offers from prospective suppliers. The system must handle at least 20 main cathegoriesand a total of 200 subcategories. The system should be based on standard software (e.g. Greatstore or similar). This means that the freelancer/supplier should be able to customize a site for us including all design and functional elements.