I try to only buy names in one of two situations. Either the name is cheap enough (less than $50) and I can find a use for it even if the traffic doesn't pan out (developed minisite), or I get full stats from the seller and verify they are legit with the parking company prior to purchase. Even then I expect some falshood in the transaction. For example I expect the seller likely provided stats for his best month or months and in reality the name probably does half on an ordinary month. Its not entirely honest or dishonest, but its human nature so you just have to plan on and account for it in your offer.
Alexa, pr, ect are all just indicators, and all have their own good and bad points. I wouldn't say don't use them. I would say don't put blind faith in them.