On retainers, we can store items. Sometimes, when you don't know better, or in a rush to empty up your inventory, you store a valueless item on them. Then sometime later, you could see it wasting space on your retainer, and be like "I should NPC this. But my inventory is full!" and then forget about it.
My suggestion is a command to tell your retainer to sell to NPC (The NPCs are like right there by the bell!). Feel free to have it be a time delay on selling the goods if you want, because if that extra two gil is a big thing for you, I think you may have other problems.
Since you can only buy from market boards, I don't think there'd be a large issue with quick buying and reselling stuff that vendors for more than the item MB cost. Again, even with the MB, the NPCs are RIGHT THERE. If you do that already, it'll still probably be faster to go to the NPC than call the retainer anyways.
Yes, I am incredibly lazy and forgetful.