You must ask the owner to show a proof of income and anything else that proofs of traffic and rankings etc.
I suggest ask a screen share also to look at in thoroughly
you could check alexa rankings.
It depends on where the traffic is coming from, ask him to let you access his google analytics. If he say no go awayhe has give me his amazon associated earning screenshot and it seems legit.
But i am worrying that will i still be able to get the amount of traffic he is getting .