it's extremely difficult to sex a silkie before a few months.. i have a few. The 7$ will probably come with a guarantee that they will replace one if it does turn out to be a rooster. I am no expert, but when young, it may require a vet to sex them out.
if you want a baby hen silkie, it's worth it..
-joe-
I have to agree with MPC...i ordered 2 female silkie chicks, white and buff, and they threw in an extra blue silkie chick as a packing peanut. Thus far, all signs are pointing in the right directions as far as accuracy goes. And that little blue silkie cockerel is the most beautiful of them all! I just LOVE them!