Official comment
@Tom > Unfortunately when a DJ decides to sell their controller, they're not obligated to sell the rekordbox license with it, as they may keep it to use with a CDJ & DJM (for example). If the user intended on giving you the license, they would have to deactivate it from their computer(s) first, then you can activate it on your own account.
If they are unwilling to deactivate it or sell it, we can't force-deactivate it from their account, you would have to purchase a license or subscription for yourself.