Carfax and other similar reports only contain information that is either in the public domain (like DMV records and police reports)and other information that is either volunteered by previous owners or paid service providers. There is no legal requirement that any service, accident, finance or other information be provided by anyone.
About the only information any owner would provide would be positive. No one would intentionally allow negative information to get into a report that could determine whether or not a vehicle would sell or have an influence on the price.
Personally I think the money would be better spent on a thorough inspection by a competent mechanic.