But a police clampdown aims to put the brakes on second-hand merchants profiting from the theft and sale of stolen vehicles and their parts.

The Altona North regional response unit has launched Operation Venus, targeting Brimbank, Hobsons Bay, Maribyrnong, Wyndham and Melton.

At least seven cars are stolen from the west each day, totalling more than 2700 a year. Although many are found dumped, a high percentage end up in scrap metal yards.

Detective Sergeant Steve Daniliuk said the practice was widespread.

"A lot of older-model cars aren't being stolen for parts; it's more likely they're scrapped for their value in scrap metal.

"Cars between 1980 and 2000 are disappearing from railway stations and houses. Metal prices are high, catalytic converters' [prices] are astronomical, so we're clamping down on whoever we see selling those items."

He said broken-down cars left unattended were being picked up by thieves in tow trucks and traded.

The organised motor vehicle theft squad will assist Venus officers to reduce the number of stolen cars being accepted.

"If [dealers] buy second-hand cars, they must have a licence and must record details such as the VIN, engine number or registration," Detective Sergeant Daniliuk said.

"They're multimillion-dollar businesses and we're trying to ensure they're monitored.

"There cars are coming in on a Monday and by Tuesday they're crushed and gone."

Merchants at industrial precincts in Laverton North and Sunshine will be among those targeted.

Detective Sergeant Daniliuk said people who legitimately purchased cars to do up and sell could only legally do so to four vehicles per year before obtaining a motor car trader licence.

There are fines of up to $10,000 for breaches.

Australian Metal Recycling Industry Association secretary Paul Ryan said worldwide demand for scrap metal, steel, aluminium and copper was rising.

Scrap copper sells for up to $6 a kilogram, or $6000 a tonne, and scrap steel for about $200

a tonne.

Two people have been charged with the theft of 11 vehicles following a police raid on July 17.

A 30-year-old Sunshine West man and a 27-year-old Werribee South woman face multiple charges, including theft of cars and handling stolen goods.

Police seized two stolen vehicles at the man's house.

The other vehicles were located at a scrap metal yard in Laverton North.