Sorry Mammabuck but maybe the seller is slightly pulling your legs - It could be a pure silkie but somewhere down the line the parents/grandparents/etc could have him mixed. Does it look like the others?
I do not think the seller is pulling your leg; it is far more likely that she is uninformed, and has birds that originated from a hatchery (either they or their parents). THere are many breeds with yellow legs, so I would not automatically assume cochin. Look at the wing feathers--are they coming in silkied or not? If not, then you have at best a silkie cross. However, it might well be another breed entirely. Do you have photos?
Oh my little girl will be disappointed...she spent her allowance to buy 4 pure bred silkies to raise as show chickens/pets/breeding stock....now she is doubting the pedigree of all her chicks.....I contacted the lady and she assured me that they are pure...but she is also selling silkie x and won't exchange for a different chick