I good detailer can take those scratches and bumps out for a $100-200. A good wash, claybar cleaning, touchup paint, sand flat and buff out it they will be barely noticable. Even a good firm rubbing with Meguiars scratch-X 2.0 would lighten them out substantially.
I always DIY fix anything a potential customer would notice on inspection so I don't have to explain anything about the car. it's 100% clean and 100% working.
Whatever the dealer is offering you is a massive discount off of what they can turn right around and sell it for. They would probably list it for $22-25K and let people talk them down to 20-21k.
Take a look at carmax selling prices for a good guide on what the market is for the car. you can get a number between the carmax (non negotiable) price and 15K and pocket the difference.
I know you were posting for sale and not asking advice but these are my 2 cents anyway.