It really depends on where you are looking to sell and with which dealership. I've heard stories of some salespeople starting at 16,17 and 18 but that's rare. Keep in mind, if you are asking people to part with 20-50k of their hard earned money, they usually want to deal with someone with a decent amount of their own life experience.

That being said, don't let anyone tell you that you can't achieve something. Keep visiting dealerships and asking for an opportunity until someone says yes. The car business can be very rewarding if you put enough effort into honing your skills.

Without a doubt a professional looking webs page is important, however I would also advise a complete e-commerce site. Today's world is a combination of brick and mortar and an online presence. One no longer survives without the other.