Online Auction

I'm looking for a script or engine that can be customized. We are
building an site that has a specialized auction as part of it's

Somethings that will be different from a regular Dutch auction are
-multiple bidders will be accepted (grouped to complete the bid) as
they will be bidding on lending and borrowing
-Auctioneer will have the right to accept or reject bids
-Auctioneer will be able to set the duration of the auction and can
close it at anytime
-Both parties can negotiate counter offers/bids
-Auction will automatically end when the total requested bid is meet

These are not features we need to be include (though any would be
nice) rather we would like to start with a base environment that can
be modified