Here is a list of checks and documents needed to change a vehicle owner in Spain
The vehicle MUST NOT be in a Statutory Off Road status (Baja/SORN), this is mandatory.
The vehicle MUST HAVE up to date ROAD TAX (can be paid at SUMA office or local town hall, it depends on your region), this is mandatory.
Valid ITV (Since 2013 it’s not mandatory, see note below).
Insurance (It's not mandatory, see note below).
NOTE: It is mandatory to have a vehicle insured before being able to pass an ITV test in Spain however you are able to sell a vehicle and perform the change of name legally without having insurance and/or ITV, the new owner's obligation will be to pass ITV and insure the vehicle before driving on public roads.
Ficha técnica (Mandatory).
Permiso de circulación (Mandatory).
Insurance receipt (Recommended)
Road tax payment receipt.(Recommended)
NOTE: The existence of a "Reserva de dominio", "Embargo", or a "Precinto" on a vehicle may lead to certain limitations when making the transfer or notification of sale, it's HIGHLY UNRECOMMENDED to purchase a vehicle with any of the above issues if you don't know what you are doing.
SALES AGREEMENT BETWEEN INDIVIDUALS
(Contrato de compraventa de un vehículo usado entre particulares)
DOWNLOAD HERE (Official DGT sources, it must be in Spanish for legal reasons).
BUYER / SELLER DOCUMENTS
If Spanish national:
DNI (Documento nacional de identidad).
Padrón (Only in the case that your actual address in Spain is different from your DNI).
If Resident in Spain:
EU ID or Passport.
Padrón (Only in the case that your actual address in Spain is different from your Residencia document.).
If Resident in Spain (With TIE Card):
Padrón (Only in the case that your actual address in Spain is different from your TIE).
If Non-resident in Spain:
EU ID or Passport.
NIE number document.
Padrón (or) property rental contract (or) deeds of property.