If your Wii U and 3DS have the same NNID, this is what you can do:
0. change the email address on your NNID to a temporary one and remove as much personal information as possible
1. borrow a second 3DS (or buy from a place with a good return policy)
2. Create a new NNID on the new 3DS
3. transfer the new NNID to the old 3ds. All games will now be associated with the new NNID. All saves will be wiped, so you will be busy playing games again.
4. Delete all friends from your old NNID, give them your new one if desired.
5. Give the buyer the NNID password and ensure they change the email address to theirs.
You now have your NNID with your 3DS games, and the buyer has the NNID associated with the Wii U games. This is the best you can do, short of completely wiping the Wii U and losing all the paid games. You will have to make sure that the buyer understands that the games are linked to the ID. They may be able to talk to nintendo and ask for a username change, but I doubt it.
Also, I checked with Nintendo and they can transfer your game collection to a new ID. I'd say talk to them, as you might be able to create a temporary NNID and they will transfer the WII U game licenses to it. Then you can give the console with a fresh ID and not lose any of your 3DS saves.