This happens a lot.  Nothing can be done if items already sold.    But, Yes not meant to list in both places at the same time.

Blacklisting does not stop you being able to see their listings. It stops them being able to hid on your listings.

I have encountered this.  I have on my auctions that I go by the NZ Post maps as to whether people are rural or not. I give the buyer the option of sending the item to a Non Rural location. If buyer still insists they are not rural and refuse to pay extra postage I just cancel the trade.

Just use NZ Post website directly.  Its cheaper.

I have purchased a car from Whanganui and I  am in Tirau before.  My son has bought 1 in South Island and himself in the North Island.   With mine I asked if he would hold it till we got down there. (we  hoped on a plane). no issues.  My son asked the seller if he would take it for a inspection at his cost. Also when he purchased  it he paid half then the car was going on a transporter to get here for him to pay the other half.

Andrew Only NZ Post deliver to Post Boxes.   So Yes you are correct there.  But you can get parcels set to you Post Box.

Very rarely NZ Post have lost any of mine.  If they have not been scanned in, they usually turn up.  The odd one I have had lost NZ Post have come through with compensation.  This can take a while. (also not the full amount you have sold the item for).

Its defiantly easier to use when you have many listings.  

Do you use My Products? Trade Vine/My Products, you can set different options for shipping.

Hi.   If they expect payment straight away then, they should use PING.