CHICAGO (CBS) — A man has been setting up meetings to buy items from people in the Auburn-Gresham neighborhood and then robbing them at gunpoint, police said Thursday.

The man sets up meetings through the Letgo buy-and-sell app and through Facebook Messenger, police said.

Once the people who think they are selling the man the items arrive, the man pulls a gun and demands their property, before fleeing back into an apartment building in the 7800 block of South Marshfield Avenue, police said.

Police said the incidents happened at 5:12 p.m. Monday, Sept. 9, and at 9:15 a.m. Friday, Sept. 13.

The suspect was described as a black male between the ages of 18 and 23, standing 5 feet 5 to 5 feet 7 inches tall and weighing 140 to 150 pounds, with short black hair and a medium complexion.

Anyone with information was asked to call the Area South Detective Division at (312) 747-8273.