Problems with car buyers

What I really don’t want to do with Omega technology.I sold my old omi on 8.4.05. The buyer assured me to re-register the car within 7 working days, and wanted to get confirmation of the re-registration, he lives only 3 houses on.I have not got anything yet.What should I do now?Once again the seller is too stupid for me.Or you should go to the police and file a complaint there for…what do I know.Or I should go to the registration office myself. but don’t have any papers from the car anymore.Help!!!!!