I think its more of the fact that enough people don't play this game. People looking to sell don't wan't to get screwed so they have their prices too high and most people looking to buy want a good deal. Because of this we have to use this antiquated system of personally contacting the person and haggling out a deal. This is not like the real world where there is trillions of dollars in the marketplace and a near unlimited supply of people buying and selling.
On top of that, I would guess most people already have their main/boring/standard gear loomed up (ex. kuyak and dgs) so they aren't looking to sell and noone is looking to buy it. That leaves "rare"/interesting items like a goedendag, bar mace, etc that when people are looking to get rid of them, they feel entitled to large sums of gold
