Wii MotionPlus vs. Wii Remote Plus : is there a difference in quality or gameplay

There is no difference except for the colour.

I'm not familiar with the actual prices, but one thing to pay attention to is whether or not you're looking at a normal Wii Remote or the Wii Remote Plus that includes the MotionPlus sensor. If that's not it, then the price difference you're seeing is simply due to the "novelty" of the black controller.

Wii systems tend to be less problematic when purchased used, since Wii mods and hardware failures are a bit more rare compared to their install base. It's still possible to get a lemon, or a modified console, so do your research ahead of time.

Wii remotes tend to take a decent amount of abuse as well, owing to the fact that many of the games are aimed at an audience that might tend to be rough on the controllers, and they're intended to be flung around. I've had some bad experiences with used controllers, but I've managed to get good service at GameStop when issues came up.

GameStop publishes their return policy online, which states that used consoles can be returned for your money back within 7 days, and can be exchanged for an identical item within 30 days, if you save your receipt. This is the US policy, and there's a chance that it is enforced differently at your local store, so make sure you confirm this with the local GameStop before you make a purchase.

GameStop can't/won't do an exhaustive check of a system before accepting it for trade; doing so would cause excessive delays for the customer. They'll make sure it turns on and loads a game properly, and that the buttons on the controllers are in working order. Make sure you put it through its paces the first few weeks so that you can confirm it's in good shape before your exchange window runs out.

If you can, you might wait a bit and see if you can get a sale. In the run up towards Christmas, and especially post-Christmas, there are likely to be some good deals.