I would just add that I have made a few transactions (buyer) on GZ and never a problem. Naturally I look at how long the seller has been a member, and maybe review some posts, but I have always had a sense that most regular GZ members are of a slightly higher caliber than the general population. There is a form of selection that takes place in this community. 

As far as NZ Post goes, I have always preferred that to courier where possible and I have never been disappointed. Tracking does seem sensible at a minimum, though that is wisdom from hindsight.