Well, the reason they're being offered at such a great deal is that you are taking the risk of not knowing the sex of the ones you are choosing. If you really only want pullets you may have to pay more for professional hatchery-sexed chicks, or pay more for older birds whose sex is more evident.
But even with hatchery-sexed chicks there is still a chance for error. I tell folks to always have a Plan B for any unwanted roos, and to not name or get attached to new chicks until you know for certain you will be keeping them. You can enjoy your chicks while they're young but resist the temptation to start knitting each of them their own Christmas stocking. Practice saying "If this one's a hen we will keep then we will name her..."
That being said, you may still be able to guess the sex of those chicks with some accuracy. At one month the males *should* start showing some more pink/reddish color in their combs, and their shanks (the skin-covered parts of their legs) *should* look thicker than their sisters. This is by no means a fool-proof method, some cockerel chicks are slower to show these characteristics, some pullet chicks appear more robust than their sisters.
A BR/Marans cross cockerel should turn out nice & meaty, he'd be a good deal at $2 anyway. Well worth your while to learn how to process for your table!
