A way, like clicking the right mouse button on an item, to sell a specific item from the soldier equip screen, at the base.
That would be especially useful for
A) When you sold everything, equip some soldier and realize you forgot something.
B) Just quickly selling a single item from the equip screen.
C) Mods with a huge amount of items, categories, items per category, a confusing naming and/or a confusing ordering. Piratez would be a candidate.
D) People who prefer graphics over lists of text.
Buying would be nice too, but there at least there is the mechanism to automatically buy depleted equipment after a combat.