That door ding has been bugging you every time you walk up to your car. Fixing it before you list the vehicle is usually the fastest way to get your asking price back, since private buyers read body damage as a sign of missed engine maintenance. Spring and early summer tend to bring out the highest demand at local dent shops, so aiming for late fall or winter often gets you a softer quote and faster service.
When you are ready to book the shop, keep the repair cost separate from any dealer negotiations you are juggling. A dent repair typically runs about $450 out the door for a standard panel. Spreading that specific cash out over time takes the sting out of the prep work while you get the car ready to sell.
Keep in mind that the intro window is a deadline, not free money, and any remaining balance left over when those 18 months are up will start getting hit with regular interest charges.