Its been declining for ages with Trade Me's changes and test weekends. I have tried all the things Callum has suggested, it helped a little but not great. These changes that are for casual sellers/buyers may just be the nail in the coffin even though many casual sellers/buyers are not reading the changes correctly.

Hi.  It will not be Trade Me directly saying your Rural. It will be up to the Courier Company Delivery Maps.   Aramex has different Rural areas to NZ Post. So while you may not be Rural  with NZ Post you might be classed as Rural with Aramex or vis versa.

Ask then for a Non Rural Address to send it to.  Or I just cancel the Trade.

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.

You agree to the shipping costs when you buy.  Unless you ask the seller questions about shipping first. 

No do not send until the payment has cleared in your account, except if it is paid by Ping or After pay.

I do all my order's first thing in the morning. My Courier picks up between 8 and 9 am. That way when the buyer is reading their tracking email the  items are usually on their way.  I do tell the buyers if they have emailed, when their items will go out.  I also state in my listings, that when they receive their tracking numbers they can no longer add to their orders. 

If you read NZ Posts Terms and Conditions, you are not allowed to send Jewellery (excluding Costume).  

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.