Two points:

1) There will always be crazy anecdotal cases like that. I don't think society should base its laws on the most outrageous common denominator. A man has the right to own a firearm even if another man acts foolish with his own.

2) If both the mother and the daughter decided to do this, and there's no evidence of fraud or deceit, then I really don't understand what the big deal is. Weird? Yes. Troublesome? Perhaps. But immoral? I don't see how it's immoral if there is no victim.

Just because something is "wierd" or bothers you personally, doesn't mean you have the right to prohibit it via legislation. If there is no victim, then the the law ought not interfere. You can claim the young girl is the victim, maybe, but if she disagrees with you then whom exactly are you helping by making her decision illegal and throwing her or her mother or the craigslist buyer in prison?