This short step-by-step tutorial will show you how to get someone else's inventory in Vanilla Minecraft.
This will delete your inventory and delete their inventory and replace your inventory with their inventory.
You need access to the server files to do this.
Step 1: Step 1 - Getting Into the Right Folder
So the first step is to go into your save folder in your server files, and go into your world folder. Then once inside that you go into your player folder in the world folder.
Step 2: Step 2 - Knowing What Files to Edit
The next step is when you get into your players folder, to locate your own name and the person you want the inventory of.
Step 3: Step 3 - Deleting the Right Files
Now what you need to do is go to the file with your name on it and delete it.
Step 4: Step 4 - Renaming Files
The next step is to select the file that has the name of the person's inventory you want to capture and right-click it and then click rename in the menu.
Step 5: Step 5 - Renaming the File (Final Step)
Now for the final step, all you need to do is type in your name and press enter. Then when you next log in, you should have their inventory. Have fun!