I'm not asking how to do it, but my concern is if it is possible to alter this info, considering how much weight is given to screenshots of these stats when it comes time to sell or otherwise monetize a site. How reliable are they?
No - I know it is easy to manipulate screenshots, but can the logs themselves be altered so as to give a fraudulent view of the worth/traffic of a site? Many demand more than just screenshots when buying a site.
The logs are in files and it is easy to manipulate files. This is obvious too. The more info you have the better. To have information from third parties is even better. It is like in all business transactions and, at the end of the day, it is a matter of trust. Jean-Luc