Part of coming to terms with the loss of a loved one is moving his things to a new place or, in some cases, a new owner. Even if the car was transferred to you when your spouse died, you still have to do the paperwork to make it legally yours before you can sell it. Before initiating the sale of the car, be sure that it is legally yours and that it won't be contested; selling the car before probate or the reading of the will, for example, could complicate matters.
Get the proper paperwork indicating that you have inherited the car. If you were a co-owner of the car, get an Affidavit of Surviving Spouse signed and use that as your proof. If it was left in a will, the executor will need to sign a form that varies depending on the state. You can get the right form from the Register of Wills or the Surrogate in your area. These won't be necessary in all states.
Take the car title, your license, your marriage certificate, proof that you now own the vehicle, a copy of the deceased spouse's will and a copy of the death certificate to the Department of Motor Vehicles or other body that issues car titles.
Explain to the customer service representative that you need to transfer the ownership of a car from your deceased spouse's name to your name. Give her the original title and any other paperwork she requests.
Fill out the paperwork. Wait to be called to a window, if necessary.
Give the paperwork and correct fees to the person processing your request. Take the new title, naming you as owner of the vehicle.
Sell the car as usual once it is in your name.