Police said the victims in the crimes, which have occurred in East and North Oakland, have met the “seller” at a designated meeting spot and are then robbed at gunpoint.
In some cases, the victims have also been assaulted, police said.
The suspects are described as young black males between 20 and 30 years old.
Police advise using caution when responding to online classifieds, and are offering safety tips, including meeting in populated public places and avoiding carrying large amounts of cash.
If anything appears suspicious, police advise leaving immediately. In the case of a robbery, police said the victim should avoid injury by complying with the robber’s demands.
The robberies are under investigation, and Crime Stoppers of Oakland is offering up to $10,000 for information leading to arrests in the case.
Anyone with information is asked to contact Officer Eric Milina at (510) 238-3426.