How do I save this in a datastore?

----------------------------------------------------- BUYING/CHECKING HANDLER[LOCAL SCRIPT] ----------------------------------------------------- 
local Player = game:GetService("Players").LocalPlayer
local Cars = Player.Cars -- Folder Inside Of Player Containing BoolValues of Cars to check if bought
local Stats = Player.leaderstats
local Cash = Stats.Cash
local SpawnCar = game.ReplicatedStorage.SpawnCar
local BoughtFunc = game.ReplicatedStorage.Bought -- Remote Function
local PriceOfItem = game.ReplicatedStorage:WaitForChild("CheckPrice"):InvokeServer(script.Parent.Parent.Name)

script.Parent.MouseButton1Click:Connect(function()
    if Cars[script.Parent.Parent.Purchased.Name].Value == true then
        -- Spawn it
        SpawnCar:FireServer(script.Parent.Parent.Name)
    else
        if Cash.Value >= PriceOfItem then
           BoughtFunc:InvokeServer(script.Parent.Parent.Name,PriceOfItem) 
        end
    end
end)


----------------------------------------------------- REMOTE HANDLER[GLOBAL SCRIPT] ----------------------------------------------------- 
local SpawnCar = game.ReplicatedStorage.SpawnCar -- Remove Event
local BoughtFunc = game.ReplicatedStorage.Bought -- Remote Function
local CheckPrice = game.ReplicatedStorage.CheckPrice -- Remote Function

SpawnCar.OnServerEvent:Connect(function(CarName)
    -- Spawn The Car    
end)

BoughtFunc.OnServerInvoke = function(Player,CarName,PriceOfItem)
    local Cars = Player.Cars
    local Car = Cars[CarName]
    local Stats = Player.leaderstats
    local Cash = Stats.Cash
    Cash.Value = Cash.Value - PriceOfItem
    -- Spawn The Car
end

CheckPrice.OnServerInvoke = function(Player,CarName)
    local AllCars = game.ReplicatedStorage.Cars
    local Car = AllCars[CarName]
    local Price = Car.Price
    return Price.Value
end

----------------------------------------------------- CAR DATA[GLOBAL SCRIPT] ----------------------------------------------------- 
local DataStoreService = game:GetService("DataStoreService")
local DataStorage = DataStoreService:GetDataStore('CARDATASTORAGE')
local DataKey = "~~" 

local CarsInGame = game.ReplicatedStorage.Cars -- A Folder Where All The Cars That Exist In The Game Are

game.Players.PlayerAdded:Connect(function(player)
    local PlayerData = DataStorage:GetAsync(DataKey..player.userId) -- Players Existing Data If They Have Played Before
    if PlayerData then
        for i,v in pairs(CarsInGame:GetChildren()) do -- Will Create BoolValues For Each Car So It Can Determine If You Have Bought A Car
            local Bool = Instance.new("BoolValue")
            Bool.Name = v.Name
            Bool.Value = PlayerData[i]
            Bool.Parent = player.Cars
        end     
    else
        for i,v in pairs(CarsInGame:GetChildren()) do -- Will Create BoolValues For Each Car So It Can Determine If You Have Bought A Car
            local Bool = Instance.new("BoolValue")
            Bool.Name = v.Name
            Bool.Value = false
            Bool.Parent = player.Cars
        end 
    end
end)

game.Players.PlayerRemoving:Connect(function(player)
    local Cars = player.Cars
    local DataTable = {}
    for i,v in pairs(Cars:GetChildren()) do
       table.insert(DataTable,v.Value)     
    end
    DataStorage:SetAsync(DataKey..player.userId) -- Saves All Cars If They Are Owned Or Not
end)

You should have a boolValue named Car or something. When the player buys the car you cans set the value of it to true. Using datastores, you can save this value, letting the game know if they bought it by checking if the value is set to true.

DataStore information

Saving a value tutorial