When it gets to the point where you can only sell your items in one or two places where no one ever goes to buy things you've effectively broken the right of first-sale.
We're not at that point yet, but Amazon's reach is unparalleled and we could easily end up in a situation where private companies, whose influence in our daily lives can dwarf that of our governments, are allowed to ignore the hard won rights we have leaving us in the same position we were in before we had them.
First-sale just means that the sellers don't need permission from nintendo to sell the items everywhere.