For future reference, hatcheries who have folks trained in sexing chickens, still have a margin of error. It is something very difficult to do. I don't mean to take away from these breeders, but it is likely that people on Craigslist, if advertising a young chick as a certain gender, are going to be incorrect.
Wing sexing, feather sexing (before a cockerel actually shows male specific feathers ~ 14 to 20 weeks) , etc are also generally snake oil
The best way to nearly ENSURE you are getting a certain gender are buying verified sex linked chicks.