I think that might not be the case these days. That is the premise I went with in April/May when I was looking and every single dealer I visited when it came down to trying to agree a price and or doing any work required on the car, the answer was always the same, zero deals, the price was not negotiable and if there was no detailed service history to show when the toothed cam belt and water pump was or indeed if it had been changed, would they throw in for the asking price, nope. I was a cash buyer, with cash in my pocket, so I walked away. The dealer knew I had the cash, and were happy to watch me walk away as they claimed, they could sell the car quickly anyway, just as it was.
And they appeared to be right as well, most of the cars I saw did indeed sell within days of being listed.
I was on the verge of giving up on my selection and lowering my standards a little when I saw my current car had just been listed over a weekend, I rang had a chat with the dealer and then drove 150 miles to see the car, all the service history was there, and it was due for the cam belt etc, which the dealer was in the process of doing in his workshops, it had one or 2 minor imperfections which he said he was going to put right before he would let the car go, which he did, at great expense, so I said I'd buy the car and a deal was struck. I'm happy with it, he has good feedback and is an AA approved dealer.