Stress
This job isn't so much stressful as it is boring. Sitting all day on a tall stool, enticing busy mall-goers to stop by and take a look at your wares, wears on you after a while. Of course, if you work on commission, the stress factor can go up by about a billion percent. If you don't make that sale, you won't get paid, and at that point you're practically throwing yourself into the arms of the masses, begging for someone to buy something.
Certain types of people could be stressed out by this job because of the type of social interaction involved in it. As a salesperson, you'll be inserting yourself into the lives of others, and more often than not you'll be rejected by them. That can be stressful for the first few weeks until you get used to it.
Otherwise, the amount of stress is pretty minimal. You'll get frustrated, of course—everyone does. Customers will stress you out, your boss will stress you out, you will stress you out...but not, like, too much. Just enough to keep it interesting.