Yeah, thats an old scam, I got those few times also.
interesting, the people getting scammed are the ones who are either very greedy or very naive or new to domaining. the industry is a little bloated and lets face it, is filled with unsightly characters doing some not-so-nice things. Domain parking and squatting technically is pretty bad, I mean sitting on a domain without adding value - just holding on to it with the intention of someone coming along and paying a bundle for it is a little sketchy - but there's money to be made.
There is nothing wrong with buying/registering a domainname with intensions to sell it without developing. Besides 'sitting on a domain' is not for free since one has to pay for the renewals etc. It is not like sitting on a nice bench in the city park and holding a sign 'this place for rent'.