I can't seem to be able to buy any spells galore items on Kumos for quests, directly from the NPC shop. The NPC picture comes up and then "buy from NPC shop" button goes away, leaving only the option to buy from user shops (it shows they are in stock on Legacy Subeta if I check).

If the items are truly not in the NPC shop, then the option to purchase from the shop should not be available.

I'm using Samsung internet on mobile

Yeah, the items are not in stock on Kumos (it's been bought out a lot recently.) I agree - the option should not be available if the item is not in stock.

I wasn't sure if it's being bought out because people RS more on Kumos, or if there's an a actual problem with the way that shop restocks

It seems like it is. Spells Galore on Kumos never has items quests need. I always have to get it from user shops and not once, have I seen the NPC shop option

Tbh, I'm not sure, as I'm taking a small break from being online. I know that when I was doing quests on Kumos, a lot of quest items said it was in npc shops, but showed as always sold out

Did some quests and everything seems to be restocking in shops! I also like a lot of the changes