Crestwood Fire Chief Jon Bruce said it was not clear what caused the employee to lose control.
"People were screaming, moaning, laying on the ground," auction shopper Tom Schroeder told the Chicago Tribune. "Employees were running around telling people not to move."
Three of the victims were in critical condition with injuries including a crushed chest and legs, Bruce said. Two firefighters were hurt while helping victims, he said.
"It was pandemonium" as rescue workers made their way through several hundred people who were leaving or helping the injured, the fire chief said.
Victims were taken to hospitals. One firefighter's injury was exposure "to large amounts of blood," considered hazardous because of the possibility of infection, he said. The other firefighter suffered a back injury while lifting a patient.
A woman who answered the phone at the auction house Wednesday afternoon said the business was "not giving out any information right now." The company's Web site shows a large, open auction site where bidders can walk around among vehicles for sale.