My chickens never really cared when they get separated, I don't think it would traumatize the chicks to leave mom after they become independent of her. By that time, they should be around 6-8 weeks old and you should be able to tell which are pullets or cockerels and can sell them as such. If you still can't tell yourself you want to advertise as a straight-run which means people have a 50/50 chance of getting a hen or rooster.
Selling day-old chicks in Maine is regulated I believe, you're not allowed to sell young chicks in any group smaller than six. But older birds aren't as regulated. I still wouldn't sell anyone less than 3 chicks per group, because they are very social animals and will need a friend to help them get used to a new flock/home environment.