So the NPC’s items seem to update from every 5am of the timezone of the realm creator.
But the issue is if you log out next to a npc the next day you log in and quickly click on the NPC, you will notice he has new items that you can see for 2~3seconds. And then the screen is refreshed and it again will show a list of new items.
For example when you log out next to Finley.
The next day you log in and quickly trade/buy with him, you have 2~3seconds to quickly click/buy stuff. And if your click and not confirm right away it ends up refreshing the item list, but you can still buy the item from the page before the refresh.
So in the screenshot below I clicked on the skull in the last space before the items were refreshed. So it removed all items and generated new ones, except for the square that I clicked where the skull was. And when I hit confirm I still get the skull
I check finley/grinnich every day on all my realms, and always buy all radiant gems. Yet every day when i happen to log in next to him I often see radiant gems that then dissapear within 2~3seconds. This means when I log into the realm they already have new items, but when i talk to them and go to buy screen it then reloads all the items again.
For keyboard/mouse people this is like a mini game to quickly click the radiant gems before they are gone. But for controller people that have a harder time to go all the way to bottom right it will be a tease of the radiant gem they won’t be able to buy 