the advant

i have never used adsense on any of my sites since so far only worked with adult, but since I got a good offer on a mainstream site I decided to give it a go, but I'm a bit in doubt about it since the guy refuses to give me access to his adsense account to verify the monthly revenue, stating that he has other sites on it too, I don't know hardly anything about adsense how does it work? do I need to open a new account and change the ads with mine, but still how can I verify what he says outside of print screens... can he pass me only the account of the site i'm buying? is it something similar to how analytics works?
thanks

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