Done that...that's how we got the 12000! So basically they don't have a leg to stand on, do they??
I'm going through the same with my mum's car at the moment. Remember, the internet car sales like redbook, carsales, etc are only a guide and they cannot give a true value as they don't know what condition the car is in.
I got quotes from the internet car sales from between $7-$9.5K, but I took it to two dealers locally and two in the adjoining city and they gave values of $5K, 2x$6K and one $7K. If you take the internet values, you would expect that you are also paying for the advertising by the seller, a RWC and also the time it takes to sell and spend on proposed buyers checking the testing the car, etc. So that cost should be reduced as you will not have to go through those hassles.
I approached a second hand dealer and the vehicle-make agent (who also sold 2nd hand cars) in both towns. I would get written quotes from local agents. You would need to tell them your intentions to buy the car, as if they quoted for a trade-in, the value would be much less again.