At the moment, there is pre-owned Wii U stuff everywhere around here... systems, games, controllers, etc.
I don't expect the system or games to get all that high in value like a lot of the Saturn stuff did, especially with the games. Some of those expensive Saturn games had a very low print run and were on more fragile CD discs. Wii U discs are once again some sort of propriety format. They kind of feel like a blu-ray disc and look like the could withstand more of a beating.
It's the tablet control itself that will make it expensive. Without it, the Wii U is a brick, so as mentioned already, those that are interested probably already have one and will take good care of it. For others, it will probably be long forgotten in 15-20 years.