As with selling anything you do your homework and you get the best results. I would strongly suggest looking at a number of sites that will value your car, including Autotrader, We Buy Any Car, Motorway. Then armed with a price you know you’ll get (allow for WBAC to reduce the value due to condition, they’ll find something unless it’s completely immaculate) you can walk away from the dealer, sell it elsewhere and pay cash for the difference.
If you choose to sell privately, you’ll get more but it will take more effort and you’ll have to deal with the potential for people to waste your time.
I'm Autistic, if I say something you find offensive, please let me know, I can guarantee it was unintentional.
I'll try to give my honest opinion but am always open to learning.
Mark