There are so many websites and apps to find used or new cars that you never have to walk into a car dealership uninformed. You can check sites like Edmunds for any car's specs, like horsepower and gas mileage.
There are sites like Craigslist where you can search for cars that people are selling in your neighborhood. But, with some sites, you're never quite sure if you're getting the best deal.
You can search and search for good prices, but how do you know if it's a good deal? A car could be available on the cheap for a reason. For instance, it could have high mileage, so its low price doesn't outweigh the risk.
There is a site and app called CarGurus, from a co-founder of TripAdvisor, that specifically sets out to find the best deals in your neighborhood. When you search for cars on the site, it ranks results by comparing a car's price to its Instant Market Value, which is its estimated fair price for that car. CarGurus uses algorithms to analyze millions of car listings every night.
When you're on the site, you input the make and model of the used car you're looking for (more on new cars in a minute). Put in your ZIP code and search, and CarGurus will show your results.