I'd recommend creating a new feature module from scratch and beg, borrow and steal C# ASP.Net code from where you can find it on the web like here:
AuctionSite Starter Kit
and other sources.
IMHO - This would be quite a task involving 100+++ hours of coding before you are ready to run in production and handle all those bidders $ with no problems and errors.
Actually I did and buy/sell type site in PHP several years ago and it worked quite well. If I were to do a auction website I may want to go back to PHP on an Apache server and no CMS for performance reasons during the last 15 seconds of bidding.