Even revealing enough details, but not everything, about the flaw to convince a potential buyer would be detrimental to the seller, as the level of details required to convince would likely massively simplify the work of the buyer should they decide to try and find the flaw themselves instead of buying. And I imagine much of those potential buyers would be state actors or organized criminal groups, both of which do have researchers in house.
The way this trust issue is (mostly) solved in drugs DNM is through the platform itself acting as a escrow agent; but I suspect such a thing would not work as well with selling vulnerabilities, because the volume is much lower, for one thing (preventing a high enough volume for reputation building); the financial amounts generally higher, for another.
The real money to be made as a criminal alternative, I think, would be to exploit the flaw yourself on real life targets. For example to drop ransomware payloads; these days ransomware groups even offer franchises - they'll take, say, 15% of the ransom cut and provide assistance with laundering/exploiting the target/etc; and claim your infection in the name of their group.
