This is news primarily because putting it on Facebook made it visible to First World residents and made them feel complicit in the transaction. Otherwise, we mostly don't really care that there are child brides being auctioned off in Third World countries (etc). "Not Our Problem."
And the outrage is problematic for a variety of reasons. Historically in the US it was common for girls to get married at age 14 to 16. Deciding that age 18 is the age of legal adult consent etc is somewhat arbitrary.
That doesn't mean I approve of auctioning off young girls for marriage, but you need to look at a variety of factors and judge this in context. The practice of a bride price does two things: It compensates the parents for providing a valuable wife who will most likely never have wealth and power in her own name because that primarily goes through men in most countries the world over and the problem is more severe in less developed countries. It also is intended to insure that her husband has the capacity to provide for her.
What you see in less developed countries is less education overall and less opportunity overall and less use of effective birth control overall. So you wind up with inherent issues where getting laid at all is highly likely to lead to pregnancy and a lot of cultural practices grow out of that reality.
I'm a former full-time wife and mom. I was also one of the top three students of my graduating high school, a National Merit Scholarship winner, etc. A friend of the family once told me "Everyone expected you to be a millionaire by the age of 30. What the hell happened?" And I pointed to my kids and said "That happened." basically.
So I've spent a lot of years reading up on women's issues and history and so forth, trying to figure out why in the hell I never got the two career couple lifestyle that I expected to have and that I felt was my due in life. And it's led me to think long and hard on what I have come to think of as "human sexual morality" and the myriad things that grow out of trying to resolve the issues rooted in "people hunger for sex and then that can lead to babies, disease, social drama, etc."
How do you resolve those humanely and in a manner that treats all parties well?
Bride Price is one way some cultures try to make sure that women -- who will not have careers of their own and power of their own -- are properly cared for by upstanding citizens who are productive and take care of their family because they are good men.
It can easily go weird and bad places, but selling children into slavery isn't actually the goal of having a bride price.
I would love to see a better world. I seriously doubt we get there by jumping to ugly conclusions about other cultures and assuming the absolute worst and refusing to take context into account.