Stress
This actually can be a very stressful job. You'll have sales quotas to make and if you're not selling, you'll be out of a job. Even if your boss doesn't fire you, you won't be able to live on what you're making if you're not selling much.
If it's a slow time of the year or the economy is bad, there just won't be enough customers coming in to sell anything.
Some department stores will put a lot of pressure on you not only to sell, but to push people into opening up store credit card accounts. You may not want to ask if they have a store credit card for the umpteenth time, but if you don't sell enough of these as well, you could be out like a wet boot in a rainstorm.
On the other hand, other stores are so lenient that they'll even let you play practical jokes on the staff—weird practical jokes that are borderline scary, but hey...anything to relieve the tension, right?