Inventories are shops. The shop is the inventory. Sometimes there is some cosmetics stuff surrounding an NPC that is a merchant, and there is a nice interaction that trying to pick up an item on display there will have the merchant ask if you are interested in buying that item.
Most NPCs can be traded with. I didn't check all. What difference does that make? Arhu is very, very obviously an important and non-generic NPC, and nobody should expect him to have the inventory of a generic NPC, even if you wouldn't necessarily assume he would have a full merchant type inventory.
Opinions can not be proven with facts, or anything else. You can justify or support an opinion with facts, but that does not make it not an opinion. You don't seem to have a clear understanding of the distinction between opinions and facts.
