One thing to add is that depending on the cycle in the economy, used cars go up and down in price. Sometimes they are much cheaper than new ones, and sometimes, it is ridiculous (it also depends on the make and model - you can often find a used one that's a few years old at a significant discount because the body style or model style has changed).
The smart economics book I read said that the best (i.e. most economical investment is a brand new car with as few options as possible - such as Toyota Corolla, Honda Civic, etc for example). The logic was that it is the most economic option in the long run:
1. It has warranty so you are not going out of pocket for any break/fix
2. No tires will be needed for at least 2-3 years
3. You hare reliability so you won't be missing anything or paying for tow trucks
4. If you don't have a lot of options, it keeps the upfront costs low, allowing you to pay less and keep it way past the time you pay it off.
That sounded great in practice but in reality not everyone wants to to drive those types of cars with manual seats, manual doors/windows, and cloth seats. We as humans tend to splurge or want to get something fancier.
One thing about business school is that you won't likely put a lot of miles on your car - you will love close to school and unless you are planning to drive it cross-country for breaks or weekends, your mileage will be pretty low. Buying a new Toyota Corolla will be a tempoporary measure unless it is your dream car and you want to keep it for 10 years (unlikely). You are going to put 5K miles a year on it and it will take some abuse being on campus and rushing around, so you will take some value out of it. While the dealer may be able to sell a used car for $14K while listing a new one at $18, a private party is likely to fetch less (due to the risk you outlined and thus a trade in is likely to be less as well - perhaps $12K?). So the cost for you ends up being $6K over a 3-year period.
I don't know what current prices are for used cars - they keep moving around but there is a case for buying something used. Perhaps 3-years or perhaps 6. Make sure you get a carfax report and don't get an old cab or something like that. However, my suggestion would be not getting another loan for your car.
P.S. I did not have much money while at school, so I went the risky route - I bought a private party 10 year-old luxury car. Could not resist the allure and cheapness of it (Luxury cars really depreciate as the costs of repair are high). It ended up working out and I drove that car for 5 more years (it cost me less than 10% of the original purchase price and I did not have to put much money into it at all - the previous single owner took a great care of it and had all service records - an important indicator of maintenance and attitude people take towards their cars).
There are some strategies but they involve a certain degree of risk and trade offs.