It'd be really cool if we could set a "default" sort of NPC shop inventory. It'd make it much more convenient to restock certain items for quests if that was the case. Being able to sort NPC inventory (preference per-shop would be ideal, since I, and many other people, use different shops for different things) would be amazing. The default would be exactly what it is now (I assume by item ID). I also realize that any other sort would introduce some sort of randomness to the list because there might be 2 or more that are the same price/whatever the sort is by, so a secondary "hidden" sort of "by ID" would still be needed to keep the order consistent.
I say "default" because having to set it every time you enter the shop or refresh the shop would mostly defeat the purpose, so it'd be a preference setting for each user, although if the user didn't set anything it would default to exactly what it is now.
Possible sort orders (both ascending and descending of course):
Price
Number in inventory
Rarity
For me, personally, number in inventory is probably the most useful but price is close behind.
So for example, if I set it to sort by price, ascending, the cheapest things in the shop would come up first (useful for people trying to restock food or something).