Screenshots are weak proof, because they can be chosen or changed. Ask for read-only access to Google Analytics and Search Console, or ask the seller to open them live with you.
Compare the income with where it really comes from: the payment processor, the ad account or the bank statements, for the past year if the site is that old.
Look at where the visitors come from. If nearly all of them come from one place, such as Google or one social account, one change there can take most of them away. A sudden jump with no reason can be bought traffic.
Make sure the seller really owns the domain, the code, the content and the customer data, and ask which accounts can legally move to you.
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