If you think that the item is worth millions instead of a few hundred of gils, buy from the competition at lower cost and sell them milions.

But that's how the market works. Personnally, when I see someone undercutting me from 1 gil, I undercut of 15% of the value and flood the market. Undercut for real or don't do it.

Making gils is not really hard, items worth millions like rare mount or minions are the worst way to make money as they are hard to farm and have few buyers.

Selling things that are craftable/collectable is the easiest way to make money. And if you want to setup a new price, just do it.