How the hell do you value a used car?

It's been a while since I've bought cars. I used to buy and sell a lot 10 or so years back and a lot has seemingly changed since then.

We're in the market for a little electric second car for my wife (I have an electric company car and have got on really well with it and I think my partner is convinced now). Especially for the mileage she does of about 50 miles to work and back each week.

However as per the title, I really don't understand the pricing on the cars we've been looking at (E208, Kona and Niro).

I've seen a lot of private sellers pricing at dealer prices and seemingly unwilling to budge but then dealer prices are reasonable competitive and I understand they now price realistically rather than expecting much haggling.

I've looked at Parker's, total car check, Ae Buy Any Car etc to get an idea of price but what would you all recommend or specifically if you found a good condition private 2021 Kona with 80-90k miles. What would you be happy paying privately and at a dealer? Those platforms seemed to indicate 8-8.5k as a rough average.

I recently found a nice private one with that sort of mileage but they didn't really want to budge past £9-9.5k and would like to see if I need a reality check more than anything and would appreciate everyone's input and thoughts on this.

Thank you :)