For starters: redbook.com.au will give you a valuation on your car. Assuming it is a manual hatch, the value they give (private sale) is $1700-3900 with and expected use of 180,000-300,000. I usually take that to mean that a $1700 car probably has done 300,000 and look like it. Sight unseen, I'm estimating the private sale value of your car at about $3000.
Is this $1500 they're offering as a trade in part of a deal that already involves a discount on the retail price?
In my experience you only trade in a car if you have lots more money than time and can afford to waste some (generally a lot). The car salesman will be paid a commission based on the total profit of the deal so is trying to get as much out of you as he possibly can. If no discount has been offered, I'd try to renegotiate the deal without the trade at $1500 off retail. Then sell the car for $3000 privately.
Don't fall victim to the pantomime that they put on - the valuer they called in to value your trade in was just another car salesman. If you went in next week the roles might be reversed. Never assume anything they tell you is true unless you have solid independent evidence. They lie for a living.
There's lots of dealers out there and at the moment the car industry is suffering a massive decline due to Covid-19. They really don't want to lose your sale.
Just my opinions.
Kim