It isn’t difficult to get wrapped up in the instant and overpay for a site, but if you do that than it’ll take you longer to be moneymaking, or you may just get taken completely and never earn money from a certain site. A you don’t do this it is important that you confirm all revenue claims the person is making about their site. At a minimum they should offer you screen shots showing their traffic counts, and their monetization efforts. Plenty of the larger flipping sites will have reasonably stringent rules that have to be followed by anyone that is listing a website for sale, these can help defend you but eventually you are answerable for safeguarding yourself. Take some time to do your own research into the niche the website is focused around. Check to see how many customer reviews each product has (this could give you an idea of how obsessed the people in this niche are about the product, more reviews often means more eagerness, and more sales).
Go to the website yourself, look round. What is your first reaction to the site? All these things will help shield you from making a bad purchase.