Don't be fooled by some of the lowest bidders, this is not an easy project and shouldn't be taken lightly. Your web site can be the most important investment that you can make in your business, make sure that it's done right. SoluDev has extensive experience in creating the web site that you have envisioned. We would like the oppurtunity to provide this service for you.
We would create a solution that would meet all of your needs. But from the description that you posted it sounds like the following functionality would be core to the project:
Administratative Interface: Allowing for account management, monitoring of all bidding as well as offerings, etc...
Seller Interface: Allowing sellers to create listing of there auctioned items and apply various settings to the specific auction, functions for completeing the sale and various other tools for managing their auctions.
Buyer Interface: Allowing buyers to place bids and/or purchase items through your site. Should also include a transaction history and other tools for managing their use of the site
Visitor Section: The portion of the website designed to bring in traffic and get visitors interested in becoming a buyer/seller
Customer Service Section: Allowing buyers/sellers/visitors to provide feedback to your organization. Also an outlet for complaints, and dispute settlement (assuming your staff will be handling that)
Please let us know how Soludev can assist you in this endeavor. We would love the oppurtunity to create this site for you. Also noted that you did not specify a website or database platform. SoluDev is a full service IT shop, we can provide you with all your needs including the web hosting that you will need for this project.