Though it is illegal to operate a car sale business without a license in California, county officials call the law almost impossible to enforce.

The dealers hang around in a nearby park, playing cards or lying on park benches, waiting for customers to call. County officials say they bring the cars in from mechanic shops and nearby dealerships, in hopes of moving vehicles that have not sold. Many of the cars have the same phone numbers listed on them. Mr. Carvajal said sellers sometimes dropped cars off on tow trucks, two or three at a time.

When the dealers saw a photographer snapping pictures of the chalked windows, they quickly wiped the prices off all the windshields. They all denied they were selling cars — one man polishing a car ran away when a reporter approached him, and five others in the park said they were waiting around for a party that would start later. One pointed to a man sleeping on a bench and said, “It’s his birthday.”

The used car markets are only the biggest and most visible of the many unlicensed industries that flourish here. On Saturday morning, men rode up and down Pacific Boulevard on bicycles, selling watermelon, papaya and cucumber out of plastic containers. At night, the street becomes a hub of prostitution, local business owners said.

Local governments have tried to combat these weekend car dealerships for more than a decade. Los Angeles briefly banned parking with a “For Sale” sign anywhere in the city. But in 2006, the ordinance was struck down in court on First Amendment grounds, and since then underground car markets in Southern California have continued to grow.