I read this often, and I guess it could be true, but those kinds of transaction would presumably go through DNM / forums like BF and the like. Which means crypto, and full anonymity. So either the buyer trusts the seller to deliver, or the seller trusts the buyer to pay. And once you reveal the particulars of a flaw, nothing prevents the buyer from running away (this actually also occurs
regularly on legal, genuine bug bounty programs - they'll patch the problem discreetly after reading the report but never follow up, never mind paying; with little recourse for the researcher).
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.