I've looked at integrating auction type software into the shopping cart system. There are problems -- one is volume. Most auction systems will work well on low volume -- and I'm categorically, and morally, against the 'last second' snipe type bid, so any software I'd develop would _not_ have that feature in it. I find it unfair to biddres and sellers, and against what auctions are all about. By removing this feature, and the transaction/locking mechanisms that it has to have, you simplify the program greatly, since an auction does not close until x-minutes after the last bid is placed. Anyway (flame off...)

For auctions, you might want to check out EasyAuction. He was looking to do some upgrades on that, but we've lost touch.

If you are trying to set up an eBay type auction, you'll need more than any free or open source package can currently handle. But, to just run a small local auction, there are several packages out there. As the technology becomes more defined, many of the packages will get simplified, which will help.

Check out http://cgi-resources.com for an overview of what software is available in each category.

http://sourceforge.net/ also has open source projects listed that are in development, and several different auction software development groups are listed.