Cars were sold from a South Auckland yard despite the company having no registration as a motor vehicle trader.
Awan Boueki Pacific Limited and its director Bakhtawar Singh, 55, were fined a record total of $250,000 for selling 69 vehicles in a year.
Individuals can legally sell six vehicles in a 12-monthly period without registration.
In sentencing, Judge Gerard Winter put the prosecution in the "most serious of cases", and the financial penalty was larger than the previous fine of $30,000 against a Waiheke Island man in July.
Karla Flood of the Ministry of Business, Innovation and Employment, which brought the prosecution, said buying from unregistered, private traders meant customers had less recourse under the law.
"Consumers who have purchased vehicles from Mr Singh do not need to take remedial action although they should be aware that they have less protection."