Need a Auction website - with multi seller backed. website and mobile applications.

Looking for a auction website and mobile (android and iphone apps) that will allow multi seller access. So to make it a auction marketplaces. Simple but high end design. Needs to work flawlessly. Can handle thousand of users at the same time. We have in house developers that will over look the project. (this is not a penny auction website) Highlights of site and a basic overview. 1. the main function of site mobile or website is opened users see a number of auctions that have very limited time on them 30sec. Every bid increases the time by 5 sec or so. very fast passed auctions. But we are limited them to only one page so may be 100 auctions per page at most. then there will be sub categories with their own auctions. 2. Website needs to have an engine that will pick auctions based on an algorithm we setup. (not all product are listed in auctions some are just left in buy it now) 3. backend needs to to very easy for buyer to sign up and have a full functional backed for sellers. 4. Back end for the website to oversee the entire webmistress seller sand buyers 5. we have some very unique ideas to add but will disclose with actual developer. We we listed so far is noting new and is being done on many site we do have a twist on the site that might make the project a bit bigger. However it is noting that can not be done. We understand that this is not a small project so only serous bidders that have experience and capabilities to build something like this. Please give us your best price!