Comentário oficial
The license key on the back of any product is not the license key to unlock rekordbox; that is provided on a separate black card within the product packaging.
If you have purchased the DDJ-RX used, the previous owner would also have to contact us to unlink their email address from the rekordbox license so it could be activated again. However, as a rekordbox license is not tied to the product, there is no obligation by the owner to also sell the rekordbox license when selling the hardware. If they have chosen to keep the rekordbox license themselves, you will have to purchase one in order to use rekordbox.
My first recommendation would be to contact the seller -- if they did agree that they were selling their rekordbox license with the controller, they'll have to request a license release using this form.
