The most recent entry in the Google Base Blog mentions that Google is experimenting with letting Google Account users pay for items posted on Google Base via credit card. Shades of PayPal and eBay.
Makes me wonder if someday they'll build a system that automatically creates AdWords ads from Google Base entries to get more people to find things for sale in Google Base and get a cut of the action that way.
Eric Giguere is the contextual advertising expert who wrote Make Easy Money with Google and Uncommon AdSense.