Yes.

In some cases when they did pay, they paid significantly less than their published rates.

From the PoV of a security researcher - why even bother disclosing responsibly (moral obligations aside)?

Best case scenario: you don't get sued into oblivion, will be ghosted and gaslightened, receive pocket change arbitrary amount of time later.

Compared to that, i suppose the exploit brokers got their stuff together - after all, time is money - chances are someone else may stumble upon the same vulnerability...

If the payout is higher priority to you than the ethics of selling an exploit that governments around the world will end up using to hunt and capture or kill political dissidents, then you are of course free to sell it on the exploit market :) I prefer to sleep at night, though.

Just to clarify, since i suppose you read that wrong: i'm not a security researcher :)