"Job of a salesman" would be uses in a normal descriptive sentence. "Job of salesman" would be useful as a label or in a list.
"Do you think you could live the life of a salesman?"
"What would it be like to do the job of a salesman?"
"Do you have any job openings this week?" (reply) "Well, we have the job of 'salesman,' the job of 'warehouseman,' and the job of 'secretary.' "
- A.