Tips on Buying Cars: Used Cars, car, buying

Question
How important is low milage on cars? I saw a dealership sell the same make and model and year of a car with 15k on it and 55k on it for nearly they same amount.

Do most people buy used cars or do most people make a lot more money than me and only get new? lol. It seems all the car reviews I find online are for new cars, and I don't know how much car models change from year to year, and I am worried about picking a year when the car wasn't worth the trouble.

Answer
Your question seems to be multiple questions. First does mileage matter on a used car?  The answer to that question is a definite "yes." I will use the analogy of a Pizza.  Is half a pizza work less than a whole pizza?  The answer is of course yes.  One way of determining the life span of an automobile is the mileage.  As the vehicle is driven farther and used more, the parts it is made from begin to wear.  The more you drive the vehicle the more the parts are worn.  The mileage should have an effect on the resale value of a vehicle.  The book value, as determined by the National Auto Dealers Association, is effected by mileage.  As an example a 2008 XYZ with 15,000 miles on it, is worth more than the same model with 55,000 miles. The value difference would vary depending on the price range of the vehicle but I would guess the difference would be about 20% less.  
A dealer can sell either of those vehicles for whatever they want, if someone will pay it.  Most Automotive dealers would not price the vehicles the same in this case but if the vehicle is a highly sought after model the price difference might be less than the book indicates.
I can't answer the question about people making more then you but I can shed some light on why people might purchase a new vehicle. One reason is that new vehicles have a window sticker, called a Monroney label. This label shows the suggested retail price for comparison purposes. Secondly, with a new vehicle there isn't any question of how much wear it has or how it's been used because it is new, thirdly, A new vehicle allows you to get exactly the options and colors that you want.
A used vehicle, with 20,000 miles, can represent a better value because if the person who owned it was gentle on wear, it might sell for 20 or thirty percent less than the comparable new vehicle but because it was gentle use it might last just as long.
if buying a used vehicle, there are many support items that could help you. Here is a short list of inexpensive support materials:
Consumer Report magazine, This magazine has an issue each year that deals specifically with used vehicles and how well they last
National Auto dealers Association www.NADA.com This will help you determine the fair market value and adjust for mileage
Kelly Blue Book KBB.com This is a another book like NADA but I personally prefer the NADA
Amazon books has a book to help car buyers "The Informed Buyer"