Mario Party DS is, as expected, a popular game. Even the atrocious console Mario Party games sell like mad, and Mario Party DS is supposed to be kind of good.
You'd think a first-party game wouldn't have a supply problem, right? Well, possibly wrong. The word is that Mario Party DS copies are flying off shelves faster than they can be restocked, leading to a minor ebay scalping wave for the title. Even Amazon.com is out of new copies right now.
If you're interested in Mario Party DS, either wait for the shortage to blow over or order online from a small retailer you trust (who probably has MSRP copies in stock). This shortage isn't going to last.
I will say, and it pains me in every part of my ass to do so, that Mario Party DS is actually pretty good. I mean, I have always hated the series ever since its N64 beginnings, and yet I gave the portable version a chance and found it to be full of personality and some actual non-mindless gameplay! The idea of the Mario team being shrunk in the story-line kind of excuses the fact that they're doing very UN-Mario like things throughout, which made sense to me, if nobody else. The graphics are rather nice for the DS, too!
So yeah, that aside, the Mario Party series tends to sell well, whether the games happen to be actually good this time or not, and they also tend to become rather buttery in the rarity department over time. Hopefully this won't be the case with the DS version because man, it barely had a chance!