The man sustained burn injuries and was rushed to a private hospital, police said.
A police official said that three men, who came on a motorcycle, sought some liquor brands from the salesman of the Tamil Nadu State Marketing Corporation (TASMAC) outlet in T Nagar.
"When he turned around to pick up the brands from the shelf, they poured petrol brought in a paint can and set fire to the shop," he said.
The incident, which happened during evening rush hour at a busy location, led to anxiety among people.
"The identity of the miscreants are being established and we are confident of arresting them soon," the official told PTI.
"Rivalry in running a bar adjacent to the liquor store could not be ruled out," he said when asked about the probable motive.