This flag could be useful on things like keys, quest items an 'garbage' loot. A shopkeeper would say default, "Oh, sorry, but I just won't pay anything for THAT!" and a builder could customize the decline string by setting a ShopDecline string on the NPC file. This string could also be used for other times the NPC refuses an item as well, perhaps?
Would like this to allow me to easily make items which still have a category and value, but are not able to be bought in shops. This would be a quick an easy way to simulate an illegal item system. For example, the key you stole from the guard's dead body is metal, and also an item, and it's worth about 40000, but it's illegal to own/sell/trade that item. [Unless you find a black market shop, which works by: (1) An NPC flag which allows a shopkeeper to override the own/sell/trade flag of an item, or (2) a conversation menu which the player must engage in negotiating the item they have for sale and the price (maybe pushing it, lol). Either way could work, and make loot hunting / selling a little more interesting, as well as some scripts to add, like if you try selling 3 + guard keys in a row to the shop keeper in Toldara, they might get suspicious and call the guards to arrest you!!]
The item value is intended for the shops which makes a value of 0 the same as the NoSell flag that you describe. A ShopDecline string is too much hassle for too little.
I don't like overriding a NoSell flag, it makes way for a way too unpredictable system. You should be able to emulate a black market shop with conversations and scripts anyhow, getting very close to what you described above.
I rather put work into allowing this with conversations and scripts.