20-Year Prospect
There's no quicker shorthand for someone who isn't being honest than "used car salesman." That's probably why they changed it to "car sales agent." That sounds much more respectable, doesn't it?
That doesn't mean it isn't a necessary job. Cars do not sell themselves, no matter what the sales agents might tell you. Any commodity will inevitably have someone there to move the process along, and in fact, all retail jobs are projected to grow over the next decade (source). Some of this is because there's a great deal of turnover, but that's across all retail. You're talking about summer jobs in a mall in the same breath as lucrative opportunities on car lots.
Because cars are big-ticket items (i.e., you don't just go to the store for chips and come home with a Lincoln Navigator), demand for them fluctuates with the economy (source). What doesn't change is the need for cars. People love them. With urban sprawl and suburbs, people need cars to get to their jobs. Cars, in some form, are not going anywhere for a long time.
Salespeople just might, though. Pretty much everything can be bought online now, and vehicles won't be too far behind (source). The fact is, while the demand for cars might increase, the demand for salespeople probably will not. Don't be surprised if in ten years, people are buying cars off of Amazon and getting them dropped off a day later.
That shouldn't be too much of a problem, though. People don't tend to stay in this position forever. Chances are, this is the kind of thing you do for a few years and either you become a manager or you move on to another opportunity.