Barry, it wasn't the amount that bothered me- I knew it would obviously be less than a private sale, say- but the dishonesty.
They gave me a "guaranteed price" and emailed me each day to say, your guarantee only lasts 7days, 6 days... etc. It wasn't guaranteed.
I was honest about all the questions, and they tried to take off £1,500, when I arrived "because it's 4 years old". Which they knew, they had the reg etc. It wasn't until a slight disagreement about what "guaranteed" meant, and getting in my car to go, that they raised it another £1000 again. No need to lie though.