From a post by Mark Baker, I started looking at Google Base - they have a lot of interesting features - easy creation, bulk upload, extensible attributes and a
predefined set of attributes.
I've only looked at the tab-delimited bulk upload, but it looks pretty reasonable so far.
For listing products, the process seems pretty easy - http://base.google.com/base/a/1214338/9143072239359280355 . Their payment system is also integrated, so Google now has a real offer listing system with decent search. I imagine the next step is to syndicate the content as xml, wrap the data in a skin and provide good indexed search.