Presumably they can't verify it, but I feel like real data would be much more likely to convert to 'sale'. Besides if that's the particular strategy the site takes then it may well develop a reputation for reliability in some sort of way.
My point is: you will always have the plausible deniability that the blackmailer has simply invented dirty data, and mixed it with real clean data
I understand and I agree. My point is just that sites make money with fake claims of viruses on your computer - I think real browsing history would scare the money out of enough unsophisticated people to make it profitable.