He was even more sceptical when the Swiss man asked not just for one, but two.
At 6m long and weighing 1.2 tonnes, the Alexander piano is believed to be the world's largest piano.
Mr Mann built the behemoth in a neighbour's shed in Timaru while he was still a secondary school pupil to satisfy his curiosity about how pianos worked.
He said the bass strings in traditional pianos were wrapped in copper to give them more mass so they would vibrate more slowly and make a lower pitch.
It meant the bass strings could be made shorter (up to 3m long), making pianos a more manageable size.
But it took away from the sound, the tone and the flexibility of the piano, he said.
None of the wires in his piano were copper-wrapped, which meant they had to be about 6m long to get the same pitch.
Their length also gave the piano a "booming, deep sound with great dynamic contrast", he said.
The piano is named after his great-great-grandfather, Alexander Barrie Mann, the first Mann to immigrate to New Zealand.
It has been played by many high-profile New Zealand pianists, including Maurice Till and Michael Houstoun, who praised its responsive dynamics and sound quality.
Video of people playing it had featured widely online and, so far, it had had about 2 million views, he said.
"Basically, the buyer in Switzerland saw my video of the Alexander piano on YouTube.
"He was one of the viewers that saw it, and he basically just sent an inquiry email saying, ‘Hey, can I buy one of these pianos, and how much will it cost?’.
"I get a lot of these crazy emails like that, so I was a bit sceptical. But this time, I followed up on the inquiry, and this time it turned out to be legitimate.
"He wants one for his house, and the other one he wants to take on a tour around European concert halls for top pianists to do concerts on.
"So he's really helping me out in a really big way.
"It’ll help advertise it to other people that may want to buy them."
He said the buyer wanted to remain anonymous for now, and Mr Mann declined to say exactly how much he was being paid to build the pianos.
"He’s paying hundreds of thousands - for each one. I mean, you can sort of get the idea.
"A normal, really high-quality grand piano like the Dunedin Town Hall Steinway, for instance, I think they go for about half a million."
On top of that, he said the buyer also had to pay for the shipping of the pianos to Switzerland.
"Yeah, the buyer must pick it up."
Mr Mann had been given five years to build the pianos, so he was working pretty long hours at the moment.
"But it's my life's dream, and I'm just living it, really."
He was so busy with the builds that he was not taking any more piano-tuning or repair clients until the Alexanders were complete.
"It's just become a really massive, fulltime job, more than a fulltime job.
"I've always wanted to build another one, and I thought maybe it's not going to actually happen. But it is now. I keep having to pinch myself."
