Another thing to be sure about is revenue, if that is why you are buying the site. If someone says it has potential but they haven't tried it I would stay away. Also when you are looking a the stats make sure that they are uniques and also check out the referer. It may be that the reason they have so many uniques is that they did some cheap pop-under advertising. They show as a lot of uniques but usually all come from one or two referers.
Remember all of the traffic was paid, and the only way to keep it up when you buy it is to continue paying.