Something like this? WIP Full Multiplayer support Grid Inventory System in UE4 - YouTube
Lets say you have a campfire actor wich need an array data of inventory.
This array need replication btw.
Player open the inventory, you store this actor on server and send initial inventory construct to build and update widgets (or whatever you want)
Player2 open and you do the same thing with player2.
If player1 put something in campfire, player1 send a request from client to server.
You remove the item from player and put to campfire
You modified the array, so replication will happen.
With OnRepNotify you can update the widgets or what you want.
Its not so simple i described, but maybe a good point to start.
With replication inventories will be always in sync 