If the phone portion works with your SIM, and it is an ATT device, then it is unlocked.
If the PIN and IMEI are truly deactivated, then there is probaly some sort of problem with the device itself which makes data impossible - for example it was reported lost or stolen, the original owners account is in arrears, etc.
If however, the seller did not cancel their BIS account with their carrier, then the PIN and IMEI could also show up as deactivated. If that is the case, then you would need to contact the seller and have him/her contact their carrier to cancel the BIS account associated with that device.
here is an article from the FAQ that might help explain it better: Buying a BlackBerry off eBay - BlackBerryFAQ