Yes, phpauction is a good one. There is an auction plugin for wordpress called WP Auctions found here. However, I tried the free version and could not get it to work. I also did not like the way it handled in the demos or appeared cross browsers. I guess it works quite well for many folks but, not for my application. I gave up on it.
I am working on integrating phpauction gpl into my wordpress install (or visa versa). User authentication issues are similar to the zencart integration. I don’t want others to post items as this will be an original art auction. So, a blog with one specific auction/seller is what I am shooting for. I would prefer that users register for the blog and auction all in one form. Not too much of a problem doing that but, to integrate aesthetically, one of the scripts will have to be modded quite heavily. Could just be on the theme end which is again no problem but, I doubt it, it’s never that easy.
I will let you know when I am successful.