It's impossible for anyone one person to control a market 100%, because the supply is infinite.

No matter what item someone else is selling--you can obtain one and sell it too. They're never have a monopoly on it. If someone is selling 380 crafted gear? You can craft some too!

Keep in mind that if someone is buying out a 70k item and relisting them at 250k. That means there's at least 180k profit margin there. It'd only be a matter of time before other smart crafters realize there's a goldmine there and will jump in. Though, that competition will still want to get around 250k~ for it. It's not often someone will see that margin. Make an item, and then post it for 70k, lol.

But over time the market will lower due to competition. Then as the profits dry up--people leave the market. Then someone can swoop in, clean house of the cheap stuff and reset the price. Welcome to economy 101! The best part about fictional economies with infinite resources is that ANYONE can play! All you need are brains and dedication.