If they (the NPA) keeps records, they will have records of the series of band numbers you purchased, so IF your bird/s were to get lost the finder would call the NPA to trace the owner, which would be you.
You also need to keep a record of your band numbers and which birds you put them on. If you sell the birds also keep records of which band numbers and birds went to the buyer, and keep their phone number and adress in case those birds get lost. Those birds would be traced back to you, and then you would know who you sold them to and could contact them.