This sounds a little odd. Either it works or it doesn't because you got it from a private sale and it is still linked to another account. PINs don't just become inactive.
yea, give a little more info as to how you came across this phone. if you bought it from someone, legitimately, call them and have them release the PIN off their account so you may use it. if not, then the phone was probably reported stolen/lost and RIM has blocked the PIN out. the only way to reinstate it is to have the original owner contact RIM and authorize it to be put back into service.
