The victim was waiting for a bus at about 3:20 p.m. near 11 El Camino Real when a driver stopped and asked him if he had any marijuana, police said.
After the victim said yes, the driver went to a nearby parking lot, and the victim got into the car to discuss the potential exchange.
The victim tried to take a small amount of marijuana out of his pocket to show the driver, and the driver brandished a handgun, police said. He stole the marijuana, a few dollars in cash and a digital music player.
The driver let the victim leave the car, and the victim ran home to his residence in Belmont.
The suspect was described as a black man in his late 20s to early 30s, 5 feet 9 inches tall and 200 pounds. He had a dark complexion, shaved head and tattoo on his lower left arm.
A female passenger was also in the car, police said. She was described as a white woman in her early to mid 20s, thin, with light brown hair worn in a ponytail. She had a tattoo of "Maurice" on the back of her neck.
The suspects were driving a 4-door, primer gray Buick that was possibly a late 80s or early 90s model.