Help with saving tables with Data Store?

So I'm trying to make a shop, and for the shop items I'm saving a data store key as a table of all the items, each of which are either true or false if they are already purchased or not. I have the following script in the button of a shop item:

button = script.Parent
price = 738
player = button.Parent.Parent.Parent.Parent.Parent.Parent.Parent

data = game:GetService("DataStoreService"):GetDataStore("MWTestData1")

function buyItem()
    print(player.Name.." is attempting to buy "..button.Parent.Name)
    data:UpdateAsync(player.userId.."Inventory",
        function(old)
            if not old then old={arm=false} end  --I'll add more items here as  I create them, but for now I just have the arm as the only purchaseable item.
            if old.arm == false then 
                print(player.Name.." hasn't bought the arm")
                if game.ServerStorage.PlayerStats[player.Name].Stats.Points.Value >= price then
                    print(player.Name.." has enough points.")
                    old.arm = true
                    game.ServerStorage.PlayerStats[player.Name].Stats.Points.Value = game.ServerStorage.PlayerStats[player.Name].Stats.Points.Value - price
                    print(player.Name.." purchased "..button.Parent.Name.."!")

                    pointskey = player.userId.."Points"
                    data:IncrementAsync(pointskey, -price)
                    points = game.ServerStorage.PlayerStats[player.Name].Stats.Points
                    points.Value = data:GetAsync(player.userId.."Points")
                    print(player.Name.."'s points: "..points.Value)
                else
                    print(player.Name.." doesn't have enough points")
                end
            else 
                print("Arm already purchased")
            end
            print(old.arm)
            return old
    end)
end

button.MouseButton1Click:connect(buyItem)

I have two problems with this: First of all, and it's a smaller problem - this will let players buy the item if their points are above 0, not above the 738 (the price). Second, and this is the real problem I have. When I buy this item, everything works just fine. However, I can keep purchasing it over and over again until I'm out of points. Then, when I leave and come back, it has saved that I've purchased it, but only if I've bought it in the previous visit. I hope I've explained the problem alright, please let me know if you need more info.

--MightyWanderer