May I ask what you bought and how you dealt with the dealer (Truecar, etc)? I may be buying another truck soon and Truecar is giving me a truly low estimated value to negotiate with.
I didn't have much room for negotiation. Trucks are really hot around here, and the price was fair considering the mileage/condition. I also needed something now, so I didn't have the luxury of walking.
The way I'd approach it is research your *local* market. Book values are generally relevant, but may not be accurate for your specific area.
Also, more philosophically, predetermine what a car is worth to you, and if you see something you like that fits your criteria, just buy it. Life's too short to stress trivialities like money. My guidelines are $2k/year for car purchase price, not including minor repairs/maintenance. That's based on buying a new $30k car, and keeping it for 15 years. I'm not a car guy. If it fits my needs, I'll drive something into the ground. Looks don't matter, it simply needs to be fit for purpose. That's why my length of ownership is so long. $30k gets you a nice fit car without lots of "luxury". You'd want to adjust your numbers to fit your wallet/philosophy.
So... getting back to my truck. I paid $10k, and it had 90k on the odo. I figure you get 200k relatively problem free miles on any quality vehicle. That puts me slightly in the hole since I drive 25k/year. I need to drive it 5 years/225k to get my value from the truck($2k/year). You'll note my numbers don't add up between new/used mileage. This truck is for commercial purposes, and gets driven more than an exclusively private duty vehicle, so adjustments were made.
All of this is to say, figure what a vehicle is worth to you, and just go with it. It doesn't matter what a book says if it doesn't reflect your needs/wants. Nobody gets the best deal except the guy that does. There's one "winner", and everyone else is a "loser".
Also, time is your friend. If you can afford to wait, do so. I also prefer private sellers, and not dealers, but you have to know a bit about mechanics, or that can go terribly wrong. Don't forget to figure in gas/time driving around looking at cars. That eats into your bottom line.
As always, the saying "perfect is the enemy of good" applies. I'd still be in a Malibu if my boss was left to the task, and that would be unacceptable. My happiness is higher this week than it's been in a long time. I'm still pissed he didn't take the Cherokee I saw seriously, and it was sold. 210k on the odo, but it was $1k, and I have parts that fit it. That's disposable money, and the 4l motor is an American classic. In retrospect, I should have just bought it, and not waited around for his opinion...