I've been giving this a bit of thought, and I think an implementation of this
would have the additional features:

1) some sort of lua file that lists items which would be 'classes' of forbidden
items (e.g. guns) -- so we we could reuse them with various npcs
2) some sort of visual indication that the item cannot be sold (graying it out?
maybe a red x in the corner?) and some sort of tool-tip text explaining why