I watch Flippa regularly and I'm amazed by the amount of fake listings showing VERIFIED Adsense revenue from what appears to be fake traffic.
These scammers, I'm assuming, are highly adept at creating dozens upon dozens of new Adsense accounts as they inevitably (eventually) get banned themselves.
One scammer was prolific, selling $50k worth of cheap sites over the course of a year, dozens of them, with 100% positive feedback and rave reviews from happy buyers. All of the traffic he had created was fake, but it was producing HIGHLY REALISTIC CLICKS that were NOT initially resulting in bans. For all I know, his personal Adsense account is still in good standing!
I suspect many of his "happy" buyers are still earning revenue and totally unaware that he's been banned from Flippa and the traffic and clicks they receive are fake (and that they are at great risk of getting banned)!
As advanced as Google's click fraud algorithms supposedly are, they're still not catching accounts that are producing $100s in revenue a month on poorly designed sites with basically zero real traffic.