The Black Market is an area underground where you can sell anything without legal prosecution. And yes, this means anything.
Location
To get to the Black Market, you will need to get into the sewers. There is an entrance near Spawn to the east. You will see a little area under the train platform with an NPC named Doug. There, you will find the sewer entrance.
How do I get one?
There are 10 Black Market stalls in the city and multiple in the surrounding towns, labeled with “bm” in the front. You can always do /as forrent and look, or check Discord for rental rights under Economy > Real-Estate.