You know that what you are doing here is illegal?
The best thing to do here is to contact the site admin again and just tell him the bug. Maybe you can ask more than 0.1btc for that.
The OP is likely trying to pull a cheap scam.
But, assuming there actually was a bug and he's telling the truth, why would that be illegal? If he reported the bug, but the site operator choose to ignore it and do nothing about it - then I'd say he has every legal and moral right to exploit it.