Building a marketplace-like website

Hi, we're two partners starting a web-based business. We need someone to build the core website functionality for us. The focus is on the architecture, not the design. The idea is to have a marketplace-like website with 3 types of users: buyers, sellers, and intermediaries. Buyers can see sellers or intermediaries near them (and adjust the radius), and sellers can upload items and tag them. Intermediaries can also request to temporarily hold items from sellers, and buyers have to send them a request to visit them at a certain time and see the item (which the intermediary can choose to then accept). Buyers can also filter the items based on these tags. Buyers and sellers should also be able to message each other in the website, and rate each other. We don't want to include too much detail in the posting, but we're happy to have a 15-30 minute call where we describe the project in more depth and scope it. We have mockups of the pages we need and of the detailed functionality - we can share after an initial call. We have some intermediate programming knowledge, so would appreciate it hearing what architecture you would use to build the website, how you would suggest we host it, etc. so we can then pick up form the initial product you deliver.