Given the secular nature of the U.S. legal system I would imagine this form of transaction to be an extremely risky investment.
Slavery is illegal. Therefore, I can only conclude that a suitor would be paying for the right to marry the girls and not for ownership rights to the girls themselves. Given such circumstances there is nothing preventing the girls from annulling the marriage or divorcing the husband after the financial transaction is completed.
Due to emancipation laws, any form of prenuptial agreement would be binding to the spouse and not her parents. Once married a child is no longer the legal responsibility of the parents and any prior arrangement concerning the future actions of the daughter would not be legally binding on the parents.
I have a lingering suspicion that the site in question is not genuine.
“Our 15 year old daughter Mary wasn’t very popular and did nothing but mope around the house bringing everybody down, so we decided to marry her off through your site. Now our house is a lot cheerier and we love our new swimming pool and Jaccuzi! We’ve told our youngest that when she turns 15 we’re going to marry her off too!"
But maybe I am merely overestimating the character of today's parent.