You will find that Google can, and has dropped PR from domains that have expired or been parked, so i would be wary of over paying for domains with PR.
Yes some people will try and scam you into making out their domain/s have PR when they don't.....terrible isn't it, You will find some of them even sell on here!
Some sellers will claim anything, whether the domain has been dropped or not, or has or hasn't PR. Some people are just scammers full stop, buyer beware is the key.