In a nutshell - if this was the business practice nobody would sell traffic. Just imagine - you charge 1000$ for traffic and you must send it till buyer earns 1000$... Why would you do that - just send it to your sites and earn that 1000$. Its even better because sometimes you may hit more than 1000$. So if it makes 1500$ for you you, you keep that 1500$. But if buyer makes 1500$ from that traffic you still get only 1000$ from him
Its impossible business practice.