NeoGAF member DSN2K claims that second-hand console purchasers are able to redownload any games that were purchased via the previous owner. From a newly created account, DSN2K was able to download Nano Assault Neo and Trine 2 at no cost.
It makes sense, given that all user accounts on a single Wii U console are able to access previously purchased content. What makes less sense is that previous purchases seem to remain connected to the hardware even after the associated accounts are deleted. Regardless, you might be in for a pleasant surprise if you pick up a used Wii U.
We've reached out to Nintendo for comment.