Step 1: create screen GUI, name it Shop
Step 2: insert frame inside the GUI, name it Shop Frame
Step 3: insert text button inside Shop frame and insert a local script
Step 4: the local script is:
----------------------------------------------------------------------------------------------
local bloxyColaId = 1352531547 -- Put the "ID" of Your "Item" that you want to sell.
local player = game.Players.LocalPlayer

function onClick()
Game:GetService("MarketplaceService"):PromptPurchase(player, bloxyColaId)
end

script.Parent.MouseButton1Click:connect(onClick)
---------------------------------------------------------------------------------------------------
Step 5:insert script in workspace
Step 5: in the script type

--------------------
--| WaitForChild |--
--------------------

-- Waits for parent.child to exist, then returns it
local function WaitForChild(parent, childName)
	assert(parent, "ERROR: WaitForChild: parent is nil")
	while not parent:FindFirstChild(childName) do parent.ChildAdded:wait() end
	return parent[childName]
end

-----------------
--| Variables |--
-----------------

local GamePassService = Game:GetService('GamePassService')
local PlayersService = Game:GetService('Players')
local InsertService = Game:GetService('InsertService')
local LightingService = Game:GetService('Lighting') --TODO: Use new data store service once that exists

local GamePassIdObject = WaitForChild(script, 'GamePassId')
local ToolAssetsToLoad = WaitForChild(script, 'ToolAssetsToLoad')

local AdminTools = LightingService:FindFirstChild('AdminTools')

-----------------
--| Functions |--
-----------------

-- Makes copies of all the admin tools and puts them in target
local function CloneAdminTools(target)
	for _, tool in pairs(AdminTools:GetChildren()) do
		local toolClone = tool:Clone()
		toolClone.Parent = target
	end
end

-- When a player with the game pass joins, give them the admin tools
local function OnPlayerAdded(player)
	if GamePassService:PlayerHasPass(player, GamePassIdObject.Value) then
		local starterGear = WaitForChild(player, 'StarterGear')
		CloneAdminTools(starterGear)
		if player.Character then -- They've already loaded and won't get their StarterGear until next spawn
			local backpack = WaitForChild(player, 'Backpack')
			CloneAdminTools(backpack)
		end
	end
end

--------------------
--| Script Logic |--
--------------------

-- Create AdminTools if it doesn't exist
if not AdminTools then
	AdminTools = Instance.new('Model')
	AdminTools.Name = 'AdminTools'

	-- Load all of the assets in ToolAssetsToLoad and put them in AdminTools
	for _, intObject in pairs(ToolAssetsToLoad:GetChildren()) do
		if intObject and intObject:IsA('IntValue') and intObject.Value then
			local assetModel = InsertService:LoadAsset(intObject.Value)
			if assetModel then
				local asset = assetModel:GetChildren()[1]
				if asset then
					asset.Parent = AdminTools
				end
			end
		end
	end

	AdminTools.Parent = LightingService
end

PlayersService.PlayerAdded:connect(OnPlayerAdded)

Step 6: insert an int. value in the script, name it gamepass id
Step 7: insert a model in the script
Step 8: insert a int. value in the model
Step 9: tell me when you are done and i teach u how to add gamepass and item :)