"If we added a feature that would enable players to search for a specific item type, it would make the high demand items sell incredibly fast. This would then result in the high demanding items completely disappearing from the market, and almost no one would find the type of items that they need."
I ALREADY can't find the type of items that I need.
Other MMO games have no problem with this at all; World of Warcraft, for instance. They manage it by letting capitalism work. Remove the price caps, and supply and demand will balance themselves.
Further, why do you let the player choose a sale price for their commodities? It's not like buyers can shop around for the lowest price because they're lucky to see even one offering of the item they want, let alone several competing offers., So almost everyone just sells at the price cap. If you allowed searching, there'd be an incentive for sellers to under-cut each others' prices.
This would also solve another problem: city advisors who keep offering to sell my items at less than the price I can get in the market. Right now this is actually a bug, since 90% of those offers are at less than fair value, and the other 10% don't profit me much more than the market would. But if market prices can swing, these offers would be worth looking at.