From my understanding items are not delted simply removed I'd asume buttons puts them in a chest for you to pick up later. But with the nmber of shops that seem to go unchecked I'd say you shouldn't have a problem
I usually delete the items. But it really depends on the status of the player. If they respond to warning or if they haven't logged in for over a month. I also take the number of items that are actually in the shop into consideration. Your shop will be fine while you're gone. Please put a reminder notice on/in your shop as well.
