I realize that mentioing another game can and does irk some of the posters here but I think it's very relevant to this thread.
SoE really needs to look at the Dark Age of Camelot selling system.
In DAoC, players buy a personal house (as we here in EQ2 have bought apartments) and can decorate and outfit them as they wish.
One of the things they can buy is a Consignment Merchant that will stand on a porch (also bought) on their personal house and whose sole purpose is a repository of all the items the player wishes to sell. The CM is 'open' 24/7/365, no matter where the player is or even if the player is offline! you can travel to a central market in each housing zone where you'll find a Market Explorer (Broker) from whom you can search for items. You do have to travel to the player's house to actually buy the item and yes it takes some time to do so but, to me anyway, that's a MUCH more player-friendly system than what SoE has graced us with.
Frankly, I have better things to do than to stand in my home (leave my computer on) for hours on end hoping someone will buy whatever it is I want to sell. That's a truly stupid way of implementing a selling system.
As far as NPC merchants buying items back from players, seems to me that a consistant system wherein the price an NPC would pay based on item condition would be best but one that tops out at perhaps 70% of the cost of making the item and that 70% is only for Pristine items. In no case should you EVER be able to sell a player-made item to an NPC for a profit. If there are such situations in the game, by all means address those but by making it across the board forbidden is really overreacting.