Right now, when one sells a piece of gear at the trading post, there is a price floor. If a piece of gear is worth 1 silver to a vendor NPC, then you cannot post that gear for anything less than 1sp 1cp on the trading post.
I am not sure how hard this price floor is. I have seen buy orders that are lower than the price floor, and I do not know if those are considered valid buy orders (if not, why are they there?) or if those are some sort of lingering bug.
I wish to propose that the price floor be removed entirely. I can see the value of retaining a warning – a warning that can be disabled in the options – that tells you whenever you are posting an item for less than the vendor price.
My guess is that the current price floor was introduced either to try to limit trading of worthless items, or to protect players from themselves. In its current incarnation, the price floor does not server either of these purposes well! If you take that price floor away, then items will be bought simply to be sold to vendors, or to be salvaged. That opens up a market for this stuff! It opens up a way to get rid of it, whereas now it simply seems to be slowly building up. It also will, in time, be a better way to deter people from posting every piece of junk they find – eventually the price of all the junk armor will drop so low that many people will consider it too much of a loss and vendor their garbage themselves. I am no economist, but I suspect this would temporarily increase item churn on the TP but dramatically reduce the total number of items on it at any single point in time. That may or may not be helpful to your stability issues – I have no idea.
The price floor also doesn’t serve much protective function right now. One could argue that it saves people from themselves. I am not sure that this was the design of the price floor, because then items would have a price floor at ~1.18 times the vendor price, instead of vendor price + 1 cp. As currently designed, when you sell items at the vendor price on the TP, you lose 5% of your money to a posting fee and another 10% to taxes if the item sells. The people posting items at vendor costs are already losing money. You haven’t protected them. You haven’t even told them, explicitly, about the 10% sales tax yet (though most of them have hopefully noticed by now). Have some faith in your players, and let them set the point where the TP is a convenient place to get rid of junk vs. where they’d prefer to go to a vendor instead.
If I have misjudged the point of the price floor, and it is actually an anti-gold-seller measure, then I still implore you to remove the price floor. Let the gold sellers try to outmaneuver the actual players who want to corner these kinds of markets! I have some faith that the players are smarter, are able to devote more time and effort to the markets as a hobby than the gold-sellers can afford as a business. A significant minority of the player base recognizes and hates gold sellers, and will make it a hobby just to try to harm them. Besides that, measures you take internally to block external gold sellers will be what makes or breaks their market – the means they use to gather gold in your game is not nearly as consequential. In fact, less gold sellers in the field annoying players with bots might have merits of its own.