According to a post on NeoGAF, buyers of used Wii U consoles will have access to any content previously owned by the old owner digitally.
This appears to be because content licenses are tied to the system, not your Nintendo Network account. This means that any users of a Wii U console that had games purchased on it can use that content, or on the flip side, Nintendo Network users can't transfer their game licenses from system to system. So even if you wipe your console clean and remove all traces of your account, your game licenses for digital purchases are still tied to it.
Currently, the only way to remove this is by sending your console to Nintendo's customer support. So far, only Nintendo of America has confirmed to do this as well.
If you're a Wii U user (or Wii User) and looking to trade in the console and have digitally purchased games, just be wary. Once you trade in the console, it looks like you'll lose all those games as well.
Source
This appears to be because content licenses are tied to the system, not your Nintendo Network account. This means that any users of a Wii U console that had games purchased on it can use that content, or on the flip side, Nintendo Network users can't transfer their game licenses from system to system. So even if you wipe your console clean and remove all traces of your account, your game licenses for digital purchases are still tied to it.
Currently, the only way to remove this is by sending your console to Nintendo's customer support. So far, only Nintendo of America has confirmed to do this as well.
If you're a Wii U user (or Wii User) and looking to trade in the console and have digitally purchased games, just be wary. Once you trade in the console, it looks like you'll lose all those games as well.
Source