This is sad, and all too common. Arranging anonymous meetings in parking lots to sell something is never a good idea, especially late at night. Some cities set up "Safe Trade Stations" at police stations to provide some security. Unfortunately, Monrovia isn't one of them. http://www.safetradestations.com/
Sounds like a case of 'no honor among thieves' to me. What normal person sells their gold necklaces at 10:30 at night? I'd put my money that this necklace comes from one of the ransacked home robberies in the past week.