Selling a car always comes down to the details, and a fresh door ding is the kind of thing that makes a private buyer try to knock five hundred dollars off your asking price. Keeping a car running well means handling little cosmetic fixes as they pop up, and spring or early summer is usually the softest time to get body work done before shops get slammed with summer road trip repairs.
You want to negotiate the dent repair separately from any other trade-in or service talks by getting a flat quote from an independent paintless dent repair tech first. Once you know the exact number, you can decide whether to pay cash or use a card window to space out the hit.
Just remember that the intro window is a strict deadline and not free money, so if any of that balance is still sitting there when the 18 months run out, standard interest charges will kick in on what is left.