The incident happened about 9.20pm.
No one was injured and the transaction concluded - with the offender leaving in the vehicle that had been advertised for sale, Lee said.
Police later received a report about the incident and a police officer spotted the vehicle parked on Karangahape Rd, in the city centre, just after midnight.
“Armed staff approached the vehicle and located the offender in the driver’s seat.”
The man was found with an imitation firearm and a large amount of cannabis inside the car.
He is due to appear in the Manukau District Court charged with possessing cannabis for supply and presenting an object like a firearm.
Police said the vehicle involved has since been returned to the registered owner.
Sign up to The Daily H, a free newsletter curated by our editors and delivered straight to your inbox every weekday.