I've always wondered this. In other games, when an untradeable piece that I can't use drops or I outgrow a piece I've been using, I normally NPC it off for a little money. Nothing spectacular, but definitely more than 1 gil. o.0
Just as an example, when the Tenfinger Tallstaff drops while I help a friend in Toto-rak, I roll my eyes because my THM is already at 50; chances are I was there for a piece for a lower job, or helping a friend. This isn't XI where I can keep it and say, "Well, maybe when a new mage class is released, I can use it again." It's just just hogging an inventory spot. And then, it's not even worth as much as a regular staff that level. Consider the NPC price almost a consolation prize for "Well, you got a drop, but not one you need."
Asking for it to sell for the market value is silly; but is it that much for NPCs to pay as much as they would for a lesser, crafted piece of the same level? I don't know about staves (and I don't feel like buying one just to find out), but the Wind Brand I got from a THM quest would NPC for 2.2k; So can't a level 30 item from a dungeon boss sell for at least 2k?