Assuming this Costco is absolutely enormous, I'd say team support would translate into neighborhoods. So Knicks fans would live in one area, Laker fans somewhere else, Bulls fans etc.
The criminal element would be the stan gangs and trolls. They would run amok and would quickly turn this new ISH-Costco refuge into a criminal haven.
Instead of Bloods and Crips there'd be the Lebron crew and the Kobe crew and they would be bitter rivals. They would occupy turf throughout the premises (certain floors) and would kill each other at moment's notice.
There'd be small petty "street" crews as well (Durant boyz, Melo squad, the CP3ers etc) but they would be in far fewer number than the Lebron and Kobe crews.
Jeff would be the mayor. The mods would be the police force and they would seldom interfere in the gang warfare btwn the stans.
There would also be a shit load of rape...
I would probably off myself to be honest...it would be a horrible place...