The Illinois Department of Natural Resources said an undercover officer responded to the ad posted by Juan DeJesus, 19, and went to the man's home Monday while posing as a potential buyer, the Chicago Tribune reported Tuesday.
DeJesus brought the alligator out of his home and was arrested on a misdemeanor possession of wildlife charge, police said.
The alligator was seized by the officer.
DeJesus is due to appear in court May 31.
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