I run a site where I get most of my "business" from unregistered users. Unregistered users are seemingly not inclined to register to use "contact seller", so I updated contact seller with a graphic random verification code (w/alternating background), and a second email verification block.
Anyway, I think you guys should consider adding this as a standard feature.
You can check it out here:
www.sailorsmarket.com/classifieds