Ehh....you'd only get paid for commissions? That's a warning sign to me. Means you could be working for a while before you actually get paid, and you're subject to the whims of the economy and people's general willingness to want to buy cars, and newer cars at that. I'd want to find out what is expected for good, typical, and low-end sales performance, and how much exactly you get paid on commissions. Spring and summer seem to be the car-buying seasons, and you're getting hired for fall and winter. I was looking for a summer job once and almost took something where I'd be getting paid on commission, and my parents talked me out of it. I've never really trusted that set-up. They'll be training you, you've arguably got some charisma, and you did well hawking for the college during phone-a-thons, but still...
On the other hand, it's a job. The weeks worth of training might end up being useful and applicable to other situations, and you *could* be good at it, you won't know until you try.
I would probably undergo the training and tell myself that I'd try it out for the next three months, or until the end of the year, to see how it works out. Selling one or two cars in that time would put a little more money in your bank account, and it'd give you some 'real-world' work experience, if nothing else. If the way you're trained makes you uncomfortable, that's a good time to back out gracefully. A few months of work seems a reasonable time to feel things out.
If everyone working that lot is paid on commission, and they're looking to hire someone new, then that at least sort-of suggests they have a desire or a need for someone new to help keep up with business. Have you asked why they decided to hire someone?