CarMax does not allow you to use a credit card to pay off negative equity. Roll the Debt Into a New Loan:

If you are only selling your car (and not buying a new one), you must pay the negative equity to CarMax on the spot.

Only accepted if the amount you owe is less than $250 .

If you are trading in your car for another vehicle at CarMax, you may be able to "roll over" the negative equity into your new auto loan.

Yes, , but you must reconcile the "negative equity"—the difference between what you owe on your loan and CarMax’s offer—before the sale can be finalized. How the Process Works

This option depends on your credit score, the value of the new car, and the lender's loan-to-value (LTV) requirements.

Cashier’s checks, certified checks, certified funds, or debit cards are accepted.