Reading the listing you can usually get a sense of problems, but there were only a few places that my BS alarm was triggered (one being that the seller claimed he made an average of $9 per day from Adsense ads that he set up just the other day. The screen shot for Adsense shows 19K impressions in three days, which seems a little off based on 2,242 Page Views Per month, with 1 Adsense block in place.)
There's no hard and fast rule to prevent things like this, but if you were to do it again, I'd advise
1) Either asking for access, or a video walkthrough of the adsense and paypal account as this is more difficult to fake.
2) Do a common sense check based on the numbers. Look at the traffic they are likely to receive from Organic Search (SEM Rush), look at the product and see if it's been sold elsewhere and look at the conversion rate you would need to achieve to justify the revenue figures quoted. As a rule of thumb, if product A costs $10 and the adword CPC is $1, I'll know that other advertisers need a conversion rate of 10% to break even. If your calculations shows this site needs more than that to produce the quoted figures approach with extreme caution.
Hope this helps
Justin