Whether you're looking to buy or sell a used car, you probably want to check its value to make sure you're getting a good deal. You could go to your local library and ask about the Kelley Blue Book or the NADA Guide or possibly even the Edmund's book, but you can also check all three online. I'm going to focus on used cars, though you can look up new cars on all three as well.
Kelley Blue Book is well-known. The book itself is small and comes out monthly. The website is updated weekly, although not every make and model will see changes that often. Prices depend on the market.
The site is easy to use. You can search in a variety of ways, but the most direct when looking for a particular car is probably by year, make, and model. KBB will show you trade-in value, private party value, suggested retail value, and certified pre-owned value. It offers explanations of each so you know which one to choose, then depending on which option you clicked, you might be asked for mileage, equipment, and condition. KBB has prices for cars about twenty years old or newer. If you need a price for an older car or if you need to know what a car was worth on a particular date, check their FAQ for the information.
NADA Guide is also well-known. The book is also small and comes out monthly (though older car values are published three times a year), and the website is updated at least that often.
Click "Pricing & Information" to get started. You'll be asked to select make, model, and year, then mileage and equipment. You'll then get rough trade-in, average trade-in, clean trade-in, and clean retail prices. For explanations, click each one and a small window will pop up.
You can find cars around twenty years old and newer. There's a tab on the top of the screen for classic cars, plus tabs for motorcycles, boats, and RVs. For historical prices, click "Help/FAQ" at the bottom of the screen. There's a link with instructions on how to obtain such information.
Edmunds may be unfamiliar, but the website is similar to the other two. They concentrate only on autos and have various tools for figuring out financing or listing a car for sale.
To start, click "Used Cars" at the top of the screen, then click "Appraise a Car," also at the top. (There is a link on this page for getting historical prices.) Choose your make, model, and year, then click "Appraise Your Car." Scroll down to input the options, then click "Get Pricing Report." The condition choices range from outstanding to damaged, so read the descriptions to figure out which one fits your car. For prices, you'll see trade-in, private party, and dealer retail values. Click each one to see an explanation.
You can also click the "True Market Value" link on the homepage and select your year, make, and model. Then click "Appraise Your Car" and enter your equipment options to get the price.
Edmunds doesn't have information on older cars (at this point, older than 1990), so it suggests looking at local ads to get an idea on price. It also does not have historical information for cars older than 2000, but if you need such information, you are directed to the Contact Us form.
So which site is the best? NADA says it has access to the most information. Edmunds focuses only on automobiles. Dealers probably will use Kelley's or NADA. All three offer Carfax reports, but read the screens carefully in case there is a fee. It really comes down to which one you feel comfortable with.