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."
Car-buyers who use registered dealers are covered by consumer law, including Consumer Guarantees Act requirements that vehicles must be of acceptable quality and free of small faults (unless they are pointed out before sale).
Registered dealers are also bound by the Fair Trading Act requirements to give correct information about the condition of a vehicle, the number of owners and the odometer reading.
Ms Flood said this week's record sentence should send a clear message to those who "actively flout the rules".
"Unregistered traders can expect to feel the full weight on the law."
The Herald could not contact Mr Singh for comment yesterday. Ms Flood said he had not appeared in the Manukau District Court for the entirety of proceedings.
There have been a number of prosecutions against those selling too many vehicles without proper registration.
# This week: Bakhtawar Singh, 55, and Awan Boueki Pacific Limited fined a total of $250,000.
# August 28: 27-year-old man fined $6000 plus court costs and $500 solicitors fees.
# August 7: 25-year-old man fined $4500 plus $500 solicitors fees.
# July 2: 37-year-old man given 350 hours community service.
# June 18: 44-year-old man fined $7500 plus court costs and $500 solicitors fees.
# May 16: 54-year-old man fined $3375 plus $500 solicitors fees.
# May 15: 35-year-old man fined $4500 plus court costs.
Source: Ministry of Business, Innovation and Employment.