I don't use Autotrader in any form at all, its pretty ineffective down in the far west. However, if you can make a living selling cars at £500 less than suggested market retail price, fundamentally CAP, I'm genuinely amazed. With cars making well over CAP clean most of the time now, I cant see where your margin is?
I personally take the approach that I price up £250-350 over CAP retail and use that as my negotiating margin. I might sell more if I was working on smaller margins, but smaller margins don't pay the bills! In effect you just end up a busy fool, chasing £500 across a car and when that car comes back to bite you, its painful.
Each to their own though.