Why do people buy babies or kids illegally from the black market ?

First, sorry if this question offends anyone. I know this is a hard subject because it’s dealing with small children. I’m asking this question out of curiosity because I saw a documentary about it.

Why would some people buy babies or kids from the black market, meaning illegally ?

Do you think some parents sell their children because they can’t take care of them, or want them to have a better life ?

Do adults who buy babies or kids give them to adoption agencies ? Do you think any adults are predators ? Do you think some people who buy kids just use them to work in sweat shops ?

What kind of people do you think would do this ?

I’m simply curious and concerned about the subject.