There's plenty of legal gray-hat companies that will buy a bug. Zerodium is a good example though it no longer exists. These companies in turn sell to NSA, etc. maybe even foreign governments for all I know. There's very little regulation of the space iirc.
I had a friend that worked in the space. He lived an exciting life; I hope he's still alive..
Ah the classic programmer mistake when encountering the legal system of thinking ‘but surely this act, in and of itself, could not be considered illegal?’ When what the law criminalizes are not mere acts but actions carried out with intent and knowledge in a context.
‘Officer, you can’t arrest me for carrying a gold bar!’
‘I can if you are carrying it out of the Federal Reserve vault without permission’
Similarly:
‘Surely selling someone the knowledge that a piece of software can be hacked isn’t illegal?’
‘It is if you know or suspect that that person’s intention is to use that knowledge to commit crimes’
This would make you a ‘coconspirator’ or possibly an ‘accessory before the fact’ in any crimes they committed.
Google is only paying for the vulnerability; the exploit market is also paying for your mortal soul.