I had a buddy (a lawyer) who ran a carfax on a 300ZX. Came back "clean" but there was an unexplained gap in the mileage.
I looked into it further and found the vehicle had been salvaged in Tx and reissued a clean title in Florida
So much for Carfax' reliability.
Not to mention that even if they do report damage thats reported to the insurance companies and police reported incidents, thats all they report. IF you DIY, then there's no paper trail