If you were not using a BES then this is only important if you were using the data features to have your email sent to your device (BIE). The PIN is associated with your email accounts on BB's side, and therefore if you do not want it to transfer to the new owner you have one of two options: call carrier to have it disassociated, or if you are moving to a new BB device it will ask you to register your pin on the network. This will replace your pin and you will not need to call anyone about it.
Also, your carrier may have a location to change device manually, for example T-Mobile does on the MyTmobile site. But it's always good to make sure on the carrier's side it is taken care of (especially if you are not moving to another BB device), and then you will like have to power cycle and they will send you fresh service books and stuff.
And shame on you for selling such a beautiful and complete device! lol