The marketplace
The Black City market is only present if the player has a number of four Trainers or more in Black City. Items sold here vary according to the current residents of Black City. Bargain Pass Powers will not affect the prices of items sold here.
Bulbapedia
It says that there is the maximum of 45 people there and everyday a person leaves untill you get to the minimum 12 but you can go to other peoples white forest via enteink where you can invite tourists to your black city get another and the shop will come back
