Try Joomla - Content Management System. There are thousands of plug ins available, a great Community Builder Tool as well as Auction Tools that can be built right into the frame of Joomla.
For an overview of all CMS just go to:
www.cmsmatrix.org , you'll find an abundance of available systems there and can compare them side by side.
Also, check at NP at the "Free" and "scripts" sections, you're bound to find something.
Best of luck,
M.
