My guess is that email doesn't work because the seller did not cancel their BIS account before they sold it to you. Therefore the PIN is still connected to the seller's account and is not available for you to use to set up your email on AT&T. Email goes through the RIM servers. not the carrier's. If that is the case, have the seller call their carrier to specifically cancel the BIS account.
Maybe this will help:
Buying a BlackBerry off eBay - BlackBerryFAQ
Even though it says eBay, the info is good when buying any used BB.