I don't think you know anything about how these industries work and should probably read some of the published books about them, like "This Is How They Tell Me The World Ends", instead of speculating in a way that will mislead people. Most purchasers of browser exploits are nation-state groups ("gray market") who are heavily incentivized not to screw the seller and would just wire some money directly, not black market sales.
I mean, you're still restricted to selling it to your own government, otherwise getting wired a cool $250k directly would raise a few red flags I think. And how many security researchers have a contact in some government-sponsored hacking company anyway? Do you really think that convincing them to buy a supposed zero-day exploit as a one-off would be easy?
Say you're in the US. I'm sure there are some CIA teams or whatever making use of Chromium exploits "off the record", but for any official business the government would just put pressure on Google directly to get what they want. So any project making use of your zero-day would be so secret that it'd be virtually impossible for you to even get in contact with anybody interested to buy it. Sure they might not try to "screw you", but it's sort of like going to the CIA and saying, "Hey would you be interested in buying this cache of illegal guns? Perhaps you could use it to arm Cuban rebels". What do you think they would respond to that?
Eh, not really? If it's a legit company who provides services to various governments, they're going to pay you, they're going to report the income to the government, you'll get a 1099 for contract/consulting, and you'll pay your taxes on the legit income. No red flags. Assuming they're legit and not currently sanctioned by the US government that is.