Ultimate Wordpress Auction plugin is the best plugin to host auctions on your wordpress site.
Plugin directory
auctions plugins
WordPress plugins for auctions. Launch any one in a live sandbox in seconds.
WP Auctions allows you to host auctions on your blog or website completely fee free!
The best description will be if I describe all functionalities that this extension can do.
Add price-drop auctions to any post type in WordPress, with automatic price reduction until a purchase is made.