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It's not often paying $2170 for a little 20c coin is considered a bargain - except when it's one of about 15 of its kind in the world.
Peter Eccles, owner of the Downtown Coin Centre, said the 20c piece was minted on to the shape of a Hong Kong $2 coin by mistake in 1975, making it one of the rarest New Zealand coins in existence.
The coin was struck when the Royal Mint was making five million 20c coins for New Zealand before it went on to strike 60 million of Hong Kong's new $2 coin, introduced that year.
It has the face of the New Zealand coin but the shape and distinctive scalloped edges of the Hong Kong one.
A coin dealer for 42 years, Mr Eccles said it was hard to figure out its true value - he knew of a few on the market in the late 1970s, but none had been sold recently.
He had set the reserve on Trade Me at $1750 and after a spate of last-minute bids it sold for $2170 to a collector in Christchurch.
Mr Eccles said the coin was in the estate of Martin Leach - a coin dealer during the 1970s - who left some of his collection to a New Zealand woman. When she died recently, her grandson found the coins in her garage, where they had sat for about 20 years.
Mr Eccles did not know how the coins came to be traded, but all had turned up in New Zealand.
"I can't imagine it would have gone into general circulation because it's so distinctive. So my theory is maybe a fortunate bank teller found it at the bottom of a bag of 20c pieces."
The find was an example of the joys of being a coin dealer.
"Especially in a country like New Zealand, you never know what's going to turn up from one day to the next. Some remarkable things turn up."
He said one couple took in an old screwed-up banknote from the pre-Reserve Bank days when the trading banks issued their own notes.
"It was worth thousands of dollars and they were thrilled."
The 1935 Waitangi Crown is NZ's most famous coin, Mr Eccles said.
"They are worth about $6000 now. But this [$2170] would be a record price for a decimal coin that was introduced for circulation, rather than being a special collector's coin.
"So for a 20c coin, that is about 10,000 times its face value. But I'd say there are maybe only 10 to 15 of them around."
