You could serialize the items like this. Take for example you have a sword and you’re trying to save it. Let’s do that while the player is being removed:
game.Players.PlayerRemoving:Connect(function(player)
-- Code
end)
Then, we’d want to make a table to save the data. We’d need to “serialize” the inventory by storing its name and its type by looping (which will be useful later):
game.Players.PlayerRemoving:Connect(function(player)
local Inventory = Player.inventory
local Data = {}
for i, item in pairs(Inventory:GetChildren()) do
if item then
-- If the item exists then we’ll add it to the table
Data[item.Name] = item.Name
end
end
end)
Then we can save it:
game.Players.PlayerRemoving:Connect(function(player)
local Inventory = player.inventory
local Data = {}
local success, errormsg = pcall(function()
-- Check if the code was ran successfully
for i, item in pairs(Inventory:GetChildren()) do
if item then
-- If the item exists then we’ll add it to the table
Data[item.Name] = item.Name
end
end
DataStore:SetAsync(Player.UserId, Data)
end)
if success then
print(“Success!”)
else
error(“There was an error saving “..Player.Name..”’s data!”, 10)
end
end)
Then in the PlayerAdded event we can load the data:
game.Players.PlayerAdded:Connect(function(Player)
Player.CharacterAdded:Connect(function(Character)
local Inventory = Instance.new(“Folder”)
Inventory.Parent = Player
Inventory.Name = “inventory”
pcall(function() -- So it doesn’t error
DataStore:UpdateAsync(function(oldvalue)
if oldvalue then
-- Add the sword value to the Player’s inventory
for i, item in pairs(oldvalue) do
local Tool = Instance.new(“StringValue”)
Tool.Parent = Player.inventory
Tool.Name = item
end
end
end)
end)
end)
Here’s the full completed script (you can instead modify the code you already have because I doubt you don’t already have a server script):
game.Players.PlayerAdded:Connect(function(Player)
Player.CharacterAdded:Connect(function(Character)
local Inventory = Instance.new(“Folder”)
Inventory.Parent = Player
Inventory.Name = “inventory”
pcall(function() -- So it doesn’t error
DataStore:UpdateAsync(function(oldvalue)
if oldvalue then
-- Add the sword value to the Player’s inventory
for i, item in pairs(oldvalue) do
local Tool = Instance.new(“StringValue”)
Tool.Parent = Player.inventory
Tool.Name = item
end
end
end)
end)
end)
game.Players.PlayerRemoving:Connect(function(player)
local Inventory = player.inventory
local Data = {}
local success, errormsg = pcall(function()
-- Check if the code was ran successfully
for i, item in pairs(Inventory:GetChildren()) do
if item then
-- If the item exists then we’ll add it to the table
Data[item.Name] = item.Name
end
end
DataStore:SetAsync(Player.UserId, Data)
end)
if success then
print(“Success!”)
else
error(“There was an error saving “..Player.Name..”’s data!”, 10)
end
end)
Hope this helps you out 
Quick note, make sure you enable Roblox API Services in your game so that the DataStore with properly work (Game Settings → Permissions → Enable API Services) 
