I believe the reference in this case is for children who were given to adoption for monetary or social gain. Some countries, such as China, it was beneficial to both the government and the mother to adopt out baby girls due to the population policy.
China doesn't have to have the expense of raising an 'orphan' or abandoned child.
The mother is can give birth to a second child, hopefully one that is male or healthy. I'm not conding this, merely explaining it. Since she can only have one child due to the government policies, it does not benefit her to keep an unhealthy child. Also, culturally, male children are preferred. So a girl child is less beneficial as well to her.
So in that case, you might argue the child was 'sold' into adoption.
Also in some South American countries, babies were stolen from their mothers in the 1980's and 'sold' to adoption brokers in the US.
Even in the US, there have been cases of pregnant women promising their babies to multiple couples. They use the baby as a way to milk these couples for money. Sometimes one couple gets the baby, the others get nothing. Sometimes no one gets the baby and the mother pockets the cash. This is a case of 'baby selling' as well.
So the concept, although oddly expressed, is possible.
I think you are referring to traditional adoption and the author is referring to somethign esle.