There is an extra bit built into the price of a car when they advertise it, they either give this off for cash or as an extra on a trade in price...
eg.
car price 9k, but we will give you 1k off for cash
car price 9k, but we will give you 1k for your 1982 escort with a ceased engine and rats living in the boot
car price 9k, but we will give you 5k for 2000 micra thats really only worth 4k