I have just done this for a friend who's husband recently died and needed to change the registration to her name. You will need to go to Traffico in person with ALL the documents relating to the car, PLUS - death certificate of the named owner in the log book, proof that whoever is selling the car actually inherited it (or is the authorised executor of the will), proof that any inheritance tax due on the car has been paid (if the seller inherited it) and all of your documents (NIE, residentia/registration, padron, inside leg measurement and anything elst which you can think of which may - or may not - be relevant. Preferably, take the seller with you and get there early in the morning.
Hope that this is of help to you.
Steve