The value of goods is based on what buyers are willing to pay and what sellers are willing to accept. Currently we have a static economy where the NPCs determine the price of those goods that they sell and buy.
For example. If Harn will pay 2,000 for a Q300 sword, then vendors will not sell these swords for any less - unless they are doing you a favor. Harn's willingness to pay this much puts a floor on the price of swords. Since he does not sell Q300's however, he does not place a ceiling on the price. Players are still at liberty to raise their prices. Last time I checked, the going PLAYER set price was about 10,000 for a q300 sabre. This price represents a balance between what vendors are willing to accept and what players are willing to pay. If Harn sold q300 swords for 8,000 then nobody in their right mind would pay any more than 8k for a sword.
Now say we had a fully dynamic economy. How would the NPC's know what price to buy and sell goods for? They would need to know what prices players have been buying and selling at. To do this, the NPC would need to be aware of all such transactions and base its price on that. But how would it do that?
Say you had it look at all transactions where items were traded for money (player to player). You could have it look at all these transactions and base its prices on these... but then players could intentionally make transactions with the intentions of driving the price up or down.... then suddenly buy or sell all their stuff in one shot and make a killing -- thus leaving the prices for these goods in an exaggerated state.
Say I drive the price of gold up to 500,000 tria per ore by BUYING MY OWN GOLD ORE from one of my ALTS or from another player account that I made for 500,000 each repeatedly. Harn sees these transactions and decides that based on the number of transactions, 500,000t must be the going price for gold. Once he does that, I take my guild house that is lined with gold ore, and sell it all to him in one shot at 500,000 per ore! Now I walk away will a huge stack of tria and leave behind Harn with his inflated prices. Likewise, I could drive the prices down so that he is selling gold for 1 tria, then jack it way up and sell all my gold.
To prevent this abuse you would need a market where the seller cannot chose the buyer. You put your merch up on the market and let buyers bid for your stuff. While it is still possible for people to buy and sell their own stuff at inflated prices, they are less likely to succeed because other players will get in between on both the buy and sell side. The fairness of this kind of market will improve with the size of the market.. but its still open to manipulation by those who can throw enough money at it to move the price.
Hope this help,
Rigwyn