Yes. What's wrong with paying people for their blood? It sounds gross, but if it works, and all parties are informed and consent, what's wrong with it?
It's exploitation. First if teenagers are actually doing this (under 18 anyway) they can't legally consent to this. Nevertheless, the only thing they are informed of is they don't have money and they want it. It's like asking what's wrong with child prostitution as long as the children are informed and consent.
You think the class of people engaging in this practice would continue if they were limited to using their own children? We aren't exactly talking about a last resort of donating an organ to keep someone alive.
This isn't really exploitation, they understand what they give up, blood and time, and what they get - money. This seems more like a trade. Do you think that any trade between two parties where one has considerably more wealth than the other is exploitation?