According to reports from gamers who purchased a pre-owned Wii U console, game licenses are registered to a specific console rather than a user account. Meaning, of course, that they can download the previous owner's games without paying a penny.
Several NeoGAF posters claim to have re-downloaded games like Trine 2: Director's Cut and Nano Assault NEO for nothing, presumably because the original owner bought the games from the Wii U marketplace before selling it on.
Currently, Nintendo Network ID accounts are also tethered to a specific console, and Nintendo have to directly step in to transfer them over to a new one (something that's currently only confirmed Stateside). This major oversight could potentially lead to some incredibly inconvenient situations should a console break or a user decides to buy a new Wii U after trading in an old one.
On the other hand, buying a used Wii U has suddenly become a lot more tempting, no?
If you can confirm this, we'd love to hear from you.