Hello,
If you are buying the full Cubase 7.5 version, then it will come bundled with a new USB-eLicenser key. You can sell your Nuendo 6 w/ NEK program to another individual and give them your current USB-eLicenser key with all of the Nuendo 6-related licenses on it.
In order to unregister your Nuendo 6 w/ NEK product from your MySteinberg account, submit a support request through your account online and provide the USB-eLicenser key number and specific product you want to unregister. We will then remove your registered products and email you back with further information. The new user will then be able to register the USB-eLicenser key number in their own MySteinberg account online.
Hope this helps!