We are able to act and to reason, as such it is reasonably expected that people will sell and buy for the amount of gold they see fit.
Forcing prices will destroy the market. Are you willing to sell your heirloom point for 400k ? I am not, and nobody is. This suggestion is among the most stupid I've ever seen.
If the average Joe wants to sell item A for 500k and no less, yet "the authority" says he can't sell it for more than 400k, what will he do ? Nothing. Killing the marketplace is not a solution, whatever the problem might be. Furthermore, I don't see what the problem is. Many people use light gear but help the team a lot with that. Heavy and expensive equipment isn't necessary better when used in "real" situations.
This might be a little too optimistic, but I do believe that every human has brains, or at least all the cRPG players. And they should be able to use a marketplace with their brains. If you are "scammed" but no bug or unexpected event happened, you were scammed by yourself for accepting a bad deal or setting up one yourself. For those that are too shy or lack information to use it, there's still the possibility of not using the marketplace. Yes ! You aren't forced to use the marketplace ! This seems to be hard to understand for some people though. Free will is hard ?
The reason why loompoint prices (heirloomed items followed) skyrocketted is because it's a tedious grind for very little gain. Anyone with even little experience of the game would prefer (from level 0 gen 1 cold start) reaching level 33 over getting 4 looms. The advantage is much bigger. That's why there are very few people selling heirloom points and that leads to very high heirloom point prices, which will lead in the future to an increase in the amount of heirloom point sellers, due to the increasing benefits of it, this increase will lower the prices, and so on. As you see, the whole thing balances itself nicely.
As I see, the main concern seems to be the manipulation of prices. Well, I don't see how anyone can manipulate prices atm. It would require an awful lot of money, and nobody has enough to even possess a tenth of all the heirloomed items for sale in the game. Any idiot knows the offers on the marketplace in great majority either are "fair" deals for both parties, or biased towards the one that set up the offer. This is pure logic. Actually, when there are two people putting similar offers, the visibility on the marketplace forces both to lower their prices until one decides then don't want to sell under a certain limit (or both agree on a price but then they will sell it slower). Visibility can not raise prizes when nothing is sold, nor can it lower them when nothing is bought. Even if prices don't please you, you have to accept them, as the entire community agreed on them. What you can do is either waiting, or taking the deal now and hoping to make more money with what you got.