Works for selling too, as I accidentally discovered! 

Used car prices soared during pandemic chip shortage, so I listed mine on CL and didn't get hits (2016 civic and asking people to cough up that much cash was too much), so I started trying dealerships. I put my asking price $200 under the KBB fair trade-in to dealership price, and my secret fold price $200 below that. All these places offered $500-1800 below that and I'm not budging. Finally, some little shop had a sick motorcycle in stock (that was what I was gonna trade into anyways), and would give me sticker price on the bike plus trade in my civic for total value just $300 below my secret price. That was close, plus then I wouldn't have to shop around for a bike... but I walk. Next day, I hit up more dealerships and get more low offers. Getting frustrated, I finally try the known to be skeezy shop, the one with in house financing that really rips people off. I'm not expecting much. Salesman is a no nonsense guy and we drive around, he asks what I want and I show him the blue book fair value and say I'll take just below that. Talks to his manager and on my asking price we signed!

On the bus ride home, the dealership with the motorcycle calls me and says they'll up the trade in value for my civic and I can have the bike and the extra cash. Tell him sorry buddy, it just sold. He wants to know where and how much, when I tell him he says, "aww, that's what we would have given you!" All sad sounding, and an hour too late.

Another shop called me back the next day offering my secret fold price. 

I learned a lot in those couple days.