Earlier quoted context omitted.
Yes. The market for 0days has been cooling off in recent years, but for a good decade there you could sell 0days, even mediocre ones for six digits. Nowdays you'll need a pretty good vuln for six digits, and something pretty stellar for seven (this isn't unheard of). ZDI, frsirt and others got into the game as middlemen. They allow(ed) you to not know who the final purchaser is and would allow you to sell 0days that…
What is the least interesting vulnerability whose sale you have firsthand knowledge of that fetched more than $20,000? Have you personally ever sold a vulnerability?
(And obviously, I don't have such a 0day to sell, so I can't prove that they would actually pay up.)