First and foremost, I'm not trying to name and shame anyone here. Just using ewamples to point out one of the limits of the market and some kind of unfairness that seems to be established.
A few select players seem to be able to
monopolize some kind of "coupon clipper service" due to how the market works. While being able to get some substantial margin, of course.
It's quite abnormal that an item can be monopolized for profit by only one player in the entire playerbase. And the said player will sure do everything they can so that no one else can ever make profit in their stead by updating their offer frequently to keep their stock up, while selling at the very minimum value possible under which undercutting makes negative profit.
Example of lucrative business: mana potions are singlehandedly monopolized by
怪盗2009, who sells at the price of 9100, making a margin of... 9 Credits by batch of 100 potions if we take the 1% fee into account. That's ridiculously low, yes, but he still has the entire playerbase for himself, at his disposal for effortless profit, with no one else ever able to contest because the market won't let anyone else sell at the same price than him, and any price lower resultats in a loss.
This issue is inherent to how the market works. The fee should be higher for items that are subject to the coupon clipper hath perk, so that nobody is able to use the perk for such a monopoly. If they want to run a coupon clipper service, that's their right and that's why I insist on not shaming such players, but at least that shouldn't be exclusive to one single player and those who want to do that need to use the forums to display their offer along with MoggleMail to trade.