There's nothing against the ToS for any of those things, but they all have their drawbacks and risks:

1 - With limited MB slots, people rarely have their entire stock of a money maker listed, so while you think you're buying it all up, they just relist more.
2 - Good practice, just make sure you check how many are needed for what people use them for and sell too small or odd quantities. If a recipe takes 9, sell in stacks of 1 or 9.
3 - If you're making a good profit, there's always going to be someone out to undercut you.