Mr2's contribution makes sense to me in light of personal experience........and the fact my brother (sadly deceased!) was a talented motor-car mechanic and lived and breathed cars. If you want to establish the parameters of the price of any particular make then get "What Car?" which lists depreciation for every make and has a section on resale values which you can calculate using mileage.
In all honesty vehicle sales-people have a pretty good idea of who they are dealing with when a potential customer comes into the showroom or their kerb-side if you happen to be buying at that end of the market. Nobody in their right mind sends someone else out to select and buy them a car. No-one wants to pay 'over the odds' especially for a high-priced item. It is necessary to be realistic and to be willing to pay the right price. Part of the price from an accredited reputable dealer is their warranties, standards of customer care and availability to you for many years after the sale. As Mr2 points out, certain dynamics in this trade may in the long run be to the detriment of the motoring public.
By the way my personal approach is to do my research, make a shortlist, test-drive all of them, choose one and go in with my cheque-book. Sorted!