Black Market…it wasn't an exact location.
Shadow Order had many secret realms, and every time, it was in a different secret realm. And no secret realm stayed in the same position.
How did Black Market came to be?
Well, Shadow Order wanted to create a trading platform of sorts for every illegal item in the federation.
Especially for the mainly expensive ones.
Why?
Easy money. For every transaction in the black market, Shadow Order had a cut.
But were the buyer and seller both fools?
Nope. If anything, they were happy that Black Market existed.
It's normal to kill the other party in these shady transactions to grab the items. But with the emergence of Shadow Order, such things were prohibited in the Black Market.
Offenders were killed without questioning.
While Black Market ate up a not so small part of profits, it became indespinsbile to shady transactions.