Go to the ANTS website: https://immatriculation.ants.gouv.fr
Click "Sell or donate your vehicle" (Vendre ou donner votre véhicule)
Who did you sell the vehicle to? "C'est une personne" select "Morale"
And then click "Acquéreur/Acheteur étranger sans numéro de SIRET"
Then you can do it without SIRET.
Fill in all the information requested, specifying that you want to declare the sale to a foreign company that does not have a SIRET.
Attach the various documents: crossed-out carte grise, proof of address, CERFA, certificate of administrative status, proof of identity.
Remember to take a picture of the crossed carte grise before pick up.
When you have done the deregistration of the car, you should have an example of the Accusé d'enregistrement Déclaration de cession, see below.
Please email us a copy of this document when you receive it.
