When buying a second hand car, what price should you pay?

No buyer expects to pay the full asking price, and no seller expects to sell for the full asking price.but what percentage should you knock off? where should you start when offering first?is the percentage different for a private seller to a garage forecourt? or is it just the starting price which is different?(just for your info, im thinking about this as im looking at buying a car which is advertised on a forecourt at £4k)
Answer
If you look up auto trader . com they have a pricing guide were you type in the make model year and other such relevant info, and they come up with a price for you its great