6'1" and only 15? No need to worry about money as the NBA contract will kick in within a few short years! Seriously, I am NOT a fan of the MTB elitest that occasionally (usually) troll these forums, but there's no way you'll get a respectable FS bike for $350, not used, not broken, pretty much not at all. I agree with the above posts about getting a new bike at your price point that is a HT (Front suspension only). It will come with at least a years worth of tune-ups and if it's a good LBS they'll teach you how to do it yourself along the way. For whatever reason bikes 1yr old hold their eBay value almost up to sticker (I have NO idea why). So buy it, learn, and if it suits you never get rid of it. If you decide at that point you need more then sell the bike and use the cash towards a good used bike since you'll now know how to maintain it yourself and also have a better idea of what to look for. It also won't hurt that by that point you'll have a better guage of what is junk and what isn't.
And to go one step further. Right now it's you "car" as you put it. In the not so distant future you'll likely have (or at least access to one) a real car and who knows at that point where your interest will drag you. Cars and chicks are the two most powerful entities in the universe. Yes, most of the time even surpassing good old money, at least at that age...