Stress
This job has an "above average" amount of stress. If you had to grade the stress, it would get a B+ for effectiveness (source). It's going to be a roller coaster ride of stressing and chilling. If you're working for an independent company on commission or are self-employed, you make your own schedule. You come in to do a couple, really expensive jobs and you could be done for the week. But the next week, there may not be any jobs.
You have to prepare for the slow days, which means working harder and longer when there is work on the table. And we all know how difficult people can be. You'll have to deal with the occasional jerk who can't understand the concept of time and how you need some in order to receive parts you do not yet have. Ah, the service industry. It is not for the faint of heart.