You'd be lucky to get much more than $200 for a used Wii U. Maybe $250 if it's the 32 GB model, but even then, considering how the thing is bundled with 1 or 2 titles at $300, it'd be tough. So it would barely pay for half of a PS4, with no games included.
Meanwhile, the Wii U currently has the best lineup of the 8th gen consoles, and is going to have at least a few big games in 2014. Even ignoring Smash Bros and Mario Kart, announced exclusives include Bayonetta 2, which is likely to be one of the best games in genre history if the original is any indicator, X, the sucessor to the best rated JRPG since the 90's, Donkey Kong Country: Tropical Freeze, the sequel to one of the more acclaimed 2D platformers of the 7th gen, SMTXFE, a crossover between two of the most acclaimed niche JRPG's, the next Zelda, a series that has never scored below 9/10 on 3D console entries, and Yarn Yoshi, another successor to one of the most acclaimed platformers of recent years. Not to mention more questionable titles like Hyrule Warriors, eShop exclusives, Virtual Console games, and multiplatform games like Watch Dogs and Call of Duty.
I dunno. Even if the Wii U seems like a poor investment now, it's never going to pay for a PS4, even if you find a generous offer. You might as well keep it and see if any of the above games turn out great. Or, alternatively, closet your Wii U for now and take it back out when a game finally comes out that's worth playing. That way, you won't miss out on said game.
Worst case likely scenario for the Wii U is that it gets abandoned by major 3rd party developers and is discontinued in 2016. That still leaves 3 full years of major releases from Nintendo, probably about 10 retail titles a year, and some nice eShop support. That seems worth the $200 for me.
Love and tolerate.
