Also, wrt Carfax...anyone can get a free CF for their own car, it's a one-time deal. So it's not unreasonable for someone to ask a seller for one, it's free. Not the case for a buyer looking at multiple cars, though. No freebie there.
As for the reliability of a CF, I've heard (true) stories of a dealer taking in a trade, running a CF that came back clean and then running it again a week later and it has something awful on it. Dealers get screwed too.
Still, they are better than nothing in most all cases and sometimes have a lot of valuable info in them. I like to see where an older car has lived it's life, more than anything. A car in a lovely car state like Oregon or North Carolina often lived its whole life in Detroit until 2 years ago, etc...