I kind of used the True Car trade in service. My bank offers it, so I uploaded some photos and details about the condition of my car and got an offer about a day later. Unfortunately the best offer was an hour and a half drive each way, which would have been a hassle. A local dealer was interested in the car, and I got them to match the Truecar price, which was about 4% more than I had negotiated from them, which was 4% over their initial offer. I might have gotten more selling it privately, but the car had some body damage that was going to cost several thousand to fix. It didn't seem like there were a lot of people interested in paying almost $10k for a damaged car (it could have easily sold for that without the damage). I had one or two leads from private buyers just under $9000, but they were a pain to deal with. The smart dealers lowballed me, offering around $5-6k. I got just over $8k for it, and I think that was a good deal for me.
Another one is Autotrader.com. They have a trade-in service that gives you an instant (or almost instant) cash offer. If you want to take the offer, just take it to the participating dealer and they will pay you as long as the car is not significantly different from the condition you put in Autotrader. It's backed by Autotrader themselves, so the dealer has nothing to lose. This service uses recent auction prices, and although Autotrader owns KBB, the price will be a little different from the KBB price.
Finally, there's Carmax. They can sometimes offer a good price for a car because they inspect them and sell them at auction. They are one of the biggest sellers of used cars at auction, so they can get good prices and buyers know what they are buying.
