And you definitely didn't take economics in school.
First, do you really want one person at SE to decide how much every item in the game is worth? Do you really think that one person is going to do a better job of balancing the entire economy than the players?
I mean, every time a new item is added to the game -- whether it's a new material, or a new item that can be crafted, or a new weapon/tool/gear -- then every price in the entire game would have to be changed. If a new rank 30 armor comes out, then the price of every other piece of r30-ish armor would go down, and the price of all the mats necessary to make those armors would go down. Mobs are constantly being changed and relocated. So if more mobs of a certain type are added or moved closer to a camp, the value of their drops needs to go down because more of them are available.
Do you think one person at SE is capable of constantly balancing those things? Considering all the other changes that need to be made to the game, I think they've got better things to do with their time than constantly monitor and balance the entire economy on every server.
One person cannot balance all of those prices better than every person playing the game. If you have one person doing it, there are going to be imbalances and loopholes all over the place. (And yes, there are imbalances in the NPC economy right now so that people can buy things from an NPC for cheap, easily convert it to something else, and sell it back to NPCs for a ridiculous profit.)
Second, how did RMTs force players to sell their scorpion claws for 30 mill? Buy up lower priced ones and sell them for 30 mill? I really, really hope that someone starts doing that on my server. I would be farming scorpion claws like crazy and selling them to RMTs for 29 mill each. The people getting rich from that type of hair-brained scheme wouldn't be RMTs.
Third, do scorpion claws still cost 30 mill? No? That's because a free market economy corrects its own imbalances. You don't need price controls.
Fourth, if you want to see the problem of having one person set all prices, just look at the OP's suggestion:
So I can sell my shield in the Market Wards -- which may take days or weeks or months -- for 900 gil (1K minus 10% tax); or I can sell my shield to an NPC and instantly get 1500 gil.currently on my server a Level 5 shield goes for 20,000 to 50,000gil
my Idea will set a minimum price of 1,000 gil that the shield is allowed to be sold at and a maximum allowing price of say 2,500 gil.
The dev team already has an NPC asking price. When you sell the shield to an NPC I do believe the NPC would buy it off of you for about 1,000 or 1,500 gil
Under that system, you can get more money faster from selling it to an NPC. So nobody would ever sell shields in the Market Wards. The only way to buy shields would be from NPCs.
In addition, I can sell a completed shield for 900 gil, or I can sell the materials that are used to create a shield for more than 900 gil. Why would I ever make a shield if I can make more money with less effort by selling the mats? Again, nobody would ever sell shields in the Market Wards.
If SE is going to set prices for everything, then you may as well get rid of the Market Wards altogether. If you want to buy or sell anything, just do it through NPCs.
Fifth, how do you propose to enforce this system? Players can still trade items individually outside of the Market Wards, for any price they want. Is a GM going to have to approve every trade in the game now?
A price controlled economy would be impossible to enforce. And it wouldn't work as well as a free market economy.