How to check if backpack slot is used?

I don't know if this will work, I will test it when I have a chance later. And this should basically like replace the item that is already in the players backpack with the item they bought.

local player = game.Players.LocalPlayer
points = player.leaderstats.Coins
Tool = false
local product = ".300 Carbine"
local prodlocation = game.ServerStorage.NewWeaponStache.Carbines -- remember to change this last part later
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function activate()
if points >= script.Parent.Parent.Price.Text then
If Tool = false then 
prodlocation:Clone().Parent = player.Backpack
prodlocation:Clone().Parent = player.StarterGear
Tool = true
Elseif points >= script.Parent.Parent.Price.Text then
If Tool = true then
local tool = playet.Backpack:GetChildren
If tool then 
tool:Destroy
Tool = false
prodlocation:Clone().Parent = player.Backpack
prodlocation:Clone().Parent = player.StarterGear
Tool = true
end
end


script.Parent.MouseButton1Click:Connect(activate)

Once again this is by guess. So I will get on pc in few and check. And if it doesn't work, I will make sure to make a work in script

Otherwise if just wanting to check for a tool like you asked. You can create a for loop checking for tools within in the player constantly.

for _,v in pairs(player.Backpack:GetChildren()) do
    if v:IsA("Tool") then -- finds tool or tools
   -- script whatever happens after a tool is found
    end
end