Law / Procedure for exporting a van from Poland to Spain? [3]

Hello.
I m waiting for information for export a van from Poland to Spain.What is the tramit i need complete for export the vehicle?
Seller says me with the van documents and the contract is OK for cross Europe to Spain and before make all the tramits in Spain...

Are there trafic offices for change the titularity of the vehicle?Are using assurance
Is there any online office for check the vehicle titularity?
Thanks