outside of the official service, there is actually one other feature that people forget exists, and would be relevent to the resell of the key.

updating by local user (no not the recently announced game sharing stuff, but the ability to update a game via just being near a device with the update)

edit: of course, this will only work if nintendo okays the transfer of the BASE game instalation as well. time will tell if its possible or not, as its a situation thats functionalyl hard to test.