It happened at around 2 p.m. in 15800 block of Arrow Highway.
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Police said the victim had posted an ad to sell a Rolex watch on the app and arranged a meeting with a potential buyer.
When the victim met with the buyer, she signaled for two juveniles to come over, according to police.
"Both suspects pointed a handgun and demanded the Rolex," read a press release issued by the Irwindale Police Department.
The woman, who was identified as 23-year-old Anabella Scott of Desert Hot Springs, and the two juveniles were later arrested and charged with robbery and assault with a deadly weapon.
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"Both juveniles were issued a citation and released to the custody of their parents with a pending future court date in juvenile court," said police.
Scott is being held at the West Covina Jail on $100,000 bail.
According to police, the watch and two handguns were found in her home during a search warrant.
The incident remains under investigation.