Carfax has been a known scam for sometime. There is no phone number you can call. They have no contact info. Their guarantee only applies if they do not provide information that was recorded by the DMV. That is the ONLY way to get your money back, and that just isn't going to happen.
Same as any other purchase, you are on your own, do some reading, there are lots of good how tos out there for buying a car. You might get dirty doing the checks, but if you are spending what could be a significant chunk of change on a vehicle, isn't it worth doing some of your own reasearch and checking?