The "original" PHPAuction was free. It was also poorly written, buggy and quite likely very insecure.
Later various commercial packages were released. Not sure if the core code was improved however. I don't believe there is a free version anymore, although you might be able to find an old copy somewhere out there. Be prepared to rework the code if you go that route.
The commercial versions come in both encrypted and non-encrypted versions. I can not attest to how they might perform.
Back in the day, I ran a site using EveryAuction (written in PERL). It was adequate for the very specific task I used it for. The software is no longer developed or supported, although you can download it still.
I have from time to time gone looking for auction software. Have yet to find anything that combines cost, functionality and security. Auction scripts are one of those things you can spend thousands of dollars on and still not know what you are getting until the shrinkwrap is peeled off the box IYKWIM.
If I were to go in this direction again I would strongly consider starting from scratch, and rolling my own package. Much of the structure of an auction package is already out there in open source land, and the specifics of accepting and tracking bidding would not be a huge task to accomplish.
(BTW - this is my 1500th post :))