I would like to know what documentation I have to provide for the change of ownership of an aircraft and whether it can be sent by registered mail.

You can download the MOD4, which contains the list of documents to be attached to that request. It can be sent by post to the address of AESA, only if the person concerned is a natural person and does not have an electronic certificate. If it is a legal person, you are obliged to communicate with the Administration telematically at this link