Carfax has what we call in science a poor negative-predictive value. If it
has a positive return (meaning it has a result, ie. turns up something bad)
the test has been useful. If it has no result (negative), all you know is
you don't know any more than when you started. Could be good, could be bad.
Trust the condition of the vehicle in front of you and your own ability to
evaluate it.
Scott