what options are there for selling a plate?

There are various methods open to you from advertising in a local or national website (e.g. Trademe) to publications such as the Trade and Exchange. 

If you have someone interested in your plate for a private sale, you will both need to complete a Transfer of Entitlement Form and send it to the Government Plate Manufacturer (details below) so they can update their records. This transfers the Entitlement of the "right to receive" the plate over to the new owner's name. The new owner will not be able to get the plates manufactured unless this form is lodged correctly.

Government Plate Manufacturer (LicenSys NZ Ltd.)
PO Box 22078

download transfer form