Stress
Because sales worker supervisors are responsible for getting others to do their jobs correctly and meeting goals, stress is naturally an issue.
Customer service is a big part of this job, and sometimes customers are messy, oblivious, or just downright rude. Dealing with them while pretending that you're not the least bit bothered can be pretty stressful.
You might also have workers who are rude to customers and other staff members. They'll stress you out...until you fire them. That'll be kind of satisfying. What won't be so satisfying is when you have to fire somebody who's sincere and hardworking, but is just plain bad at their job. You're obligated to only keep the best workers so that the company doesn't go under, but having to fire someone who's trying their best just plain stinks.
Sometimes, no matter how good your workers are, sales lag and goals are missed. At other times, your numbers may take a hit because of a strike somewhere or a natural disaster that's completely out of your control.
No matter how good the reason is for your staff not meeting their weekly or monthly goal, there's going to be a higher-up telling you that they're unhappy with the numbers. So, yeah, it's stressful, at times.
However, companies recognize the need employees have for stress management and most offer some options to help so you can continue to function on the job.