I tried Carfax for the first time a few weeks ago. Was looking at a vehicle in an auction 2 hours away, and figured Carfax would give me some useful background. Ha.
After plunking down my Visa, the report swore up and down that the vehicle was practically perfect in every way. One owner vehicle, low miles, tags last renewed ~50k km and nearly 2 years ago. That seemed odd, so I pulled the used vehicle package from the Ministry of Transportation (I'm in Ontario) for $20. Big diffrerence.
Original owner traded this Suburban to a Ford dealership 14 months ago. The Ford dealer sold it to a nondescript used lot after a month. That used lot kept it as a shop truck for a year, ran the miles up, beat it to heck, then set it out on their lot for sale when the tags ran out. When nobody'd buy the poor whupped critter, they farmed it out to piggyback in a government surplus auction, where I found it.
As far as Carfax was concerned, the last time the original owner renewed his license plates was the last time anything happened to this truck. In reality, it'd passed through 2 more owners and piled on 50k since then. They did eventually refund my Visa when I told them about the missing information, but it was an awkward process. I won't trust their data again.
Good luck!