I had no reply to my post, so I do like to inform you what is happening.
I went to the RACC in Tarragona in order to talk with the people which do pretend they can not do the register change of the car, because the buyer does not collabarate and does not pay his dues. They told me to go to the police and make a “denuncia”. The “Mossos d’Esquadra” where, to my big surprise, very kind and polite, but did not accept the “denuncia” because this is not a penal criminal case. They did send me to trafico to put the car “de bajo”. Trafico, which is as far as I know a national thing, did refuse to cancel the circulation permit, because I do not live in Tarragona anymore! So the fact is: In Spain you can sell a car, and if the buyer want, he will be able to drive the vehicle without changing the owners name at Trafico for years (In my case 2/12 years). The seller will be reponsable for all damages caused by the car and pay alt the trafic fines as well as taxes etc. What a big mess! I did contact a German lawer in Alicante. He was very kind but told me he could not do anything about this…