My old car I could've probably sold for $9k if I could be bothered waiting for the right buyer.
Trade in value was $3k.
I sold it within 24hrs of it going on carsales for 7.5k (without RWC).
If you really want to sell your car, dump the price and get rid of it. You'll still get far more than you would from a dealer.
To the original poster: IF you find the right buyer, you could probably get $10k, but like someone else mentioned, no one wants 4 door Skylines. Even if they did want one, 4 door 33's may be rare, but 4 door 34's are nearly as common as the Coupe's, and still only $12,000-14,000.