People talk about "people selling vulnerabilities" as if there's an established pattern for selling arbitrary vulnerabilities. There is not. There's an established pattern for selling exploits for RCE vulnerabilities on a subset of popular client-side platforms. It's not an especially easy market to break into (as with consulting, people starting out here tend to end up subcontracting, and taking a huge income hit).

For any other kind of vulnerability, you're not so much "selling a product" as you are "helping plan a heist".