When I had played classic there were a large group of cheating communities that were pretty large during the botting scene. I just happened to join each one and mingle between them all. Selling on multiple boards and talking to different people.
I was offered RSGP in a trade on a different forum, and had no idea where to sell it so I looked around, and came across Sythe, I sold the RSGP at a nice rate that same day.
I needed to sell an old Runescape account that was collecting edust so i googled "runescape blackmarket" and Sythe was the second link that actually had active members.