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Do you have the contract and a copy of the identity card of the seller?
If you have them you could go to the closer DGT and fill the form declaring the sell out
If you do not have the documents, you must contact the buyer and got them
There are no more legal ways to solve the problem with the City hall and DGT
I mean, you need a copy of the the identity card of the buyer
If you probe that the buyer if missing and you have lost all contact with him, may be with a writing to the DGT explaining the facts, they could inscribe the data of the buyer... but legally you will need the copy of his ID
ID for documento nacional the identidad and DGT as direccion general de trafico
You have ask for a legal answer of your problem
Its a very comun problem in Spain
You must do the change of the name in DGT when you sell out the car
Now you must have the DNI of the buyer and the contract
Its not my fault, it is how the law says it must be done
If you do not have the copy of the DNI legally there is not an answer for you