dejadeadnz:
Ultimately I think the LEGAL argument for the consumer's position is weak but the people who simply unthinkingly invite the shop to potentially duke it out with him at the Disputes Tribunal are missing one point: time is money; disputes resolution of any sort ALWAYS costs money. If the shop stands its ground and the consumer chooses to, this thing will go to the Disputes Tribunal. The relevant people at the shop may have to attend a hearing and prepare documents/statements for the tribunal before hand. It may be that you deem this worth it as a matter of principle but it's a decision to be made after a bit of reflection and not out of pique because whatever losses you might incur from settling things with the consumer may be cheaper than duking it out at the DT. Even if you win at the DT, you may not get much costs back either. This is another thing to remember.
Agree 100%.
Legally you may be totally in the right, but that doesn't mean it is necessarily worth it to 'fight it out'
The other thing I'll add to dejadeadnz's advice is hat you should consider is that even if the customer doesn't take you to the disputes tribunal, there are other ways vindictive customers can harm your business.
From something as simple as posting a negative facebook or yelp review or similar, or telling everyone they know that you are scammers, all the way to standing outside your store warning people going in that you are con artists.
They have already demonstrated their irrationality by their actions in your store.
As much as it is galling to do it, it's probably better for you to just suck it up, let them swap out their phone for the one they want, and sell the one they give you back second hand on Trademe.