For me the plausible deniability stops at something like the firm we hired (years ago, they were subsequently bought by CSID which was in turn purchased by another of my employers because it's a small world and apparently some people have unbounded appetite for risk) which steals credentials from people who steal credentials.
I can just about imagine sleeping after doing that all day. Because at least some of the time, indirectly, and after somebody gets paid, you're helping. It'd be like working in insurance sales. Your employer creams a profit off misery, but arguably the misery would be worse without them, maybe.
But any of these "Well I could get more money from bad guys" arguments miss in my opinion the most important constraint which is that most people don't want to work for the bad guys. Peanut farmer or crook is an easy question for most of us.
https://i.blackhat.com/USA-19/Wednesday/us-19-Shwartz-Sellin...
And you can always look at things like the Zerodium Price List. I don't know that it's taken very seriously in its particulars, but the general structure of it mirrors what I've heard from other sources.
You'll notice on the Zerodium list that they will pay for serverside RCE in web apps --- but only a particular kind of web app: the kind that is deployed in lots of places. National IC agencies will, for instance, pay for phpBB RCE, because they have targets that use phpBB, and there are lots of phpBB's (when you hear people talking about how valuable a web bug is, ask yourself whether that person has mentioned the weird market for phpBB bugs --- something I've had firsthand [refused!] experience with). What you won't see are bugs in SAAS applications. Again, the reason is that a phpBB vulnerability has a half-life: everyone has to install the patch once it's burned.
I have no evidence for what I'm about to say here, so take it with a grain of salt:
I assume you can get paid for a vulnerability even as esoteric as this Slack ATO bug. But you'll get paid for people who are buying Slack accounts, not Slack bugs. That is: you'll have to be the one exploiting it, and you'll be making one-off deals to use it to get targeted accounts. People sell all kinds of accounts; it would not surprise me even a little to hear that there was a market for company Slack accounts.
But to participate in that market, you'd almost certainly have to directly enter a criminal conspiracy. You wouldn't be selling a bug to a market; you'd be participating.