High quality (breeding not health) Barred Rock have some features that are good indicators of sex. Males have "runny" head spots while females spots are more precise and often smaller. Females may also have darker legs. This may be what your seller was going by. Unfortunately these chicks do not appear to be high on the breed qualities, so I'm not sure how accurate those indicators would be on these chicks. Once their barring comes in (as long as Dad was double barred and mom single barred as true barred rocks would have) The males will have a whiter appearance than hens due to double barring.