In this particular episode, there was a 'Black Market' where types of contraband were sorted in different aisles with signs marking where each type of illegal item was, in a similar manner to a real-life supermarket. Criminals went down each aisle pushing actual shopping carts.
1 Answer
Sounds like you're referring to 'The Merc,' which appeared in Gotham S02E05, "Rise of the Villains: Scarification."
From a recap of the episode:
The youngest Pike heads to the Merc, which is like the CostCo of heavy artillery. Literally – there are friendly “blue light special” announcements, and shopping carts for the big spenders.