Dealers always try to buy at bottom book price, so they can sell at a profit, after all they have to honour any warranties, and have their reputation to uphold.
We found in the past that selling privately and offering to pay cash or no part Ex, usually gave us a discount on the price of the new car.
However, most garages these days want to offer credit, and make more commission on HP than any profit on the car, so most will not entertain cash prices.
Re value, you can have a look on ebay and monitor similar aged cars and expect that yours should be worth more than this if you can sell it locally.
However many cars for sale on ebay never actually complete the sale on ebay, as most need to be viewed by the potential customer, so a private sale goes through and suddenly the car is " No long for sale" thus reducing any fees payable to ebay.
You can see this blatantly advertised as " Advertised elsewhere, so maybe withdrawn from sale at any time..."
So realistically the ebay sold prices tend to be lower than what can be achieved.
There is a web site :nenau that you can use FOC enter your reg details and a value appears.
I do question why you would trade down to an X trail when the Terrano is one of the most reliable and easily fixed 4x4's available today.
Good luck with the sale.
Best regards,
Rustic