I've used carbargains in the past and it worked very well. The service is almost identical to CarWoo's but they have been around longer.
http://www.checkbook.org/auto/carbarg.cfm
Schmidt, F.L. & Hunter, J.E. (1998) The validity and utility of selection methods in personnel psychology: Practical and theoretical implications of 85 years of research findings,” Psychological Bulletin,
Thanks. I had read the conclusion I mentioned in monographs about psychology I was reading for other research interests, and I'm glad you cited a key review article. I found the article's full text
They still need to work on their pricing. They have an opportunity to convert quite a few of their non-paying customers if they'd simply offer a plan between 2GB and 50GB.