TrueCar also has estimates on late model year cars, but other than that you will have to do your own market research to determine a fair price. Following that, you can still haggle, but within reason. A used car has fatter profit margins for a dealer (I used to sell cars and the used end was the bulk of my commissions). For used car haggling, convince yourself of a price you would be willing to pay OTD (out the door) and stick to it. If the dealer isn't willing, don't hesitate to leave.
You might notice dealers like to keep you from leaving. That's to increase pressure to buy. But that's a two sided sword, the customer can also negotiate often by being stubborn on a price, and if the dealer can make a profit, sooner or later they will agree. But don't go asking 50% off on a relatively new economy car, they can and will find a willing buyer eventually.