The key is "knowingly" sell. If they lie, and present fake documentation, then it makes it hard for you to be prosecuted.
A good example is Tracy Lords. She was 16 and 17 when she did all but one of her porn films. She had lied, and had fake documents. Although every "actor" was scared to death at first, nobody was ever charged with stat rape, child abuse or child porn. The videos were ordered to be destroyed. Now, possession of one today is illegal, but the participants didn't go to prison because there was no way they could have known she wasn't of age.
I think those producers had a better case than the GGW producers, but apparently the court in Florida doesn't think GGW should be liable.