An interesting aspect of increased user friendliness, like auction system is the idea vendors(unlike quest NPCs) can be incorporated into the UI as part of auction/trade system: selling random items and providing a minimum buying price for auctions.
All UI vendors can therefore be unified as Single "Buy Vendor Items"/"Sell at Vendor" windows, appropriate to character level or area(if item is only sold at specific area).
1.Removes trips to town. More immersion.
2.Reduces queues/crowds near NPCs
3.Much faster to sell/buy items with UI.
4.Can be set to auto-sell/auto-buy certain item classes(if game allows it), since NPC is with the player all the time(allowing to buy healing potions mid-battle).