Officer Johnna Watson said the victims are out-of-town buyers who aren't familiar with Oakland.
She said the ads usually say the car is for sale in another city but before the final sale of the car the seller changes the location of the deal to Oakland.
The buyer is then directed to the upper area of Lake Merritt, near Interstate Highway 580 and Harrison Street, Watson said.
When the buyers arrive at the designated location, they are approached and robbed at gunpoint by two to five black males, she said.
Watson said police want to remind people to use extra caution when conducting business with unknown individuals or in unfamiliar locations.
She said anyone with information should contact the Oakland Police Department's major crimes section investigators at (510) 238-3475.
Oakland police and Oakland Crime Stoppers are offering a reward of up to $20,000 for information leading to the arrest and conviction of those responsible for these crimes.