How do I make a purchase leaderboard?

I've been working on a homestore but I don't know how to actually get the leaderboard to count the purchases ever made from the store by a local player.

local Database = require(script.DatabaseService)

local GlobalDataBase = Database:GetDatabase("global")

game.Players.PlayerAdded:Connect(function(Player) local leaderstats = Instance.new('Folder') leaderstats.Name = "leaderstats" leaderstats.Parent = Player

local Rank = Instance.new("StringValue")
Rank.Name = "Rank"
Rank.Parent = leaderstats
Rank.Value = Player:GetRoleInGroup(3285600)

end)

Basically I want it so purchases made from the store in and out of the game are counted on the leaderboard where it says [Username] - [Buys] - [Rank].