It is mostly from Hollywood movies but I have read true story books and have seen news reports on people (not always spies) selling things from weapons, secrets, hacking products, to infiltration services and alike.
In real life how would someone go about meeting somebody to exchange what they have for payment?
For the ordinary Joe in the street that happens to get their hands on something, how would they sell it to a rogue state? How do people meet these high up characters to begin with to engage in these deals? Walk into an embassy and say they want to talk business?
All answers would be good if they have a source, but I am only looking for current things, so no cold war stuff unless it is somehow relevant to today.
Hopefully, this question remains a question and does not set off any alarms given the current state of the world.
Edit: I am not interested in the Dark Net. I already spent some time on there during my research. Some deals have to happen in person, and other times by publicly posting what you have you can destroy the information's value (for example saying "selling a top secret xyz product").