After a seller and buyer agree to meet on the classified ad website, the parties meet in a pre-arranged location, and the victim is held up at gunpoint, according to a community alert.
The men have stolen phones, wallets, credit cards and IDs.
One person was robbed about 3:05 p.m. Jan. 16 in the 600 block of West Surf Street. Another was robbed about 10:45 p.m. Jan. 29 in the 2400 block of North Ashland Avenue, police said.
Police describe the armed robbers as black men between the ages of 18 and 25, standing between 5-foot-5 and 5-foot-7. They may be driving a black, four-door sedan, possibly an Infiniti, police said.
Police urge people to use caution when responding to to free, anonymous "for sale" postings, according to the community alert.
Police ask anyone with information to call 312-744-8263.