The primary issue I see with that is that the player just need the full set at time they're going to purchase. The primary issue isn't that is hard to buy things, but that many things simply aren't listed because it's too time consuming for sellers to be bothered listing low value items with very small margins, for example, bone and scale fillers. This leads to more and more people leveling up tailoring and armour smithing out of necessity more than out of desire to actually craft. By making it easier to list things sellers will be more likely to list those lower value items. Even making the game remember the last listing price and giving us a relist option in our expired items would save people tonnes of time.