The mall was abandoned in a suburb of Chicago. The production company contacted a bunch of the former franchise stores located in the mall and got them to stock the empty stores with real merchandise just for the nights of the shoots. The production company agreed to pay for any and all merchandise damaged/destroyed in the filming. They also hired security to guard the merchandise when nobody was present.
As it turns out, some security people they hired were apparently stealing merchandise when nobody was around.
Landis said they had to hire additional security guards to watch the original security guards.