Sellers frequently have several custom channels set up to track individual ads on their site and they usually base their revenue estimates on the total sum of those and put them in screenshots too. I find that they frequently overlap and are generally less reliable, usually unintentionally, but sometimes on purpose, they can even include other sites in the stats. It could be something minor up to 5% or more like 50%+, but it's always a good idea to ask for url or site channel screenshot.
While screenshots can easily be faked, I found that very few sellers would cross that line, however many sellers who seem honest and trustworthy overwise may choose to display their site in a better light by omitting some relevant stats and info.
The one thing I always ask for first when considering an adsense site for purchase is a screenshot of the site channel for past 3 months. While url channels have to be set up manually and not everyone does it, site channels are created automatically for each site and are easily accessible.