If a Blackberry isn't new and you don't know the person that you are purchasing from I would be very weary.
Unlike most phones where the carrier dictates which phone can communicate on their network, in the case of Blackberries and RIM's data service, they (RIM) control the data access. If a phone is reported stolen to your carrier, who then notifies RIM there is no way you will be able to use Blackberry service anywhere in the world with that phone as its PIN would be barred.
While the PIN can be illegally changed, what would be the point as RIM keeps a record of all PINs assigned and the likelihood of you guessing a valid PIN who isn't already activated is next to nill.