Hi Turvaya, the answer to your question is literally in the next points from that quote. Hope that helps.
You mean:
3.) Was it the best option for the child?
If 12 years old is not the age of consent and the parent is not doing their best effort make decisions for their child that best promotes their safety and health, then it would be appropriate for anyone to take that child from them and to do a better job.
All options for the child are first crossed off if they violate the non-aggression principle. Since sleeping with someone below the age of consent is rape and violates the NAP, these options are crossed off first.
The first paragraph sounds like:
If nobody cares, than it is ok, to sell your child.
Nobody means, nobody in the near neighborhood. So, if my neighborhood doesn't like, when I sell my child, I just can move to a neighborhood, where nobody cares.
Also, what does "then it would be appropriate for anyone to take that child from them and to do a better job" mean?
You mean taking by force? Wouldn't that violate NAP? Can I just take a child from people, when I think, they are shitty parents?
The second paragraph seems to tell a whole different story:
Why is it rape?
If it is about fucking up their psyche and health, than I am wondering what is the difference to letting them work in a mine?