The craft can carry six people but is not allowed on the roads as, among other things, it does not have any brakes.
"We started redeveloping it in a hangar at the Rotorua Airport, but the money ran out and it went into storage for seven or eight years.
"It runs well but someone would have to do a bit more work on it if they want to use it commercially."
Mr Williamson said he would be happy to help the person who bought it with the remaining work.
"But, if you want to use it as a toy, it's fine," he said.
The craft originally ran a 13B rotary motor, which was "as noisy as hell and ran really hot. But we went back to a normal combustion engine and it's as quiet as any car."
He said the craft took a bit of work to learn how to drive but he was confident he could teach anyone the basics in an hour.
"They are a lot of fun ... it's a little bit like driving a jet boat.
"I believe it's the lightest, fully enclosed hovercraft in the world available for under US$1 million.
"And it looks like it's something that's just dropped in from outer space."
Mr Williamson can be contacted at kwkcat@xtra.co.nz, or go to www.Trade Me.co.nz and search for the listing under "hovercraft".