How Do I sell something from the in-game leader-board and make it into a different currency?

First of all you'll need to track Player Deaths.

Players.PlayerAdded:Connect(function(Player)
	local Leaderstats = Instance.new("Folder")
	Leaderstats.Name = "leaderstats"
	local Deaths = Instance.new("IntValue")
	Deaths.Name = "Deaths"
	Deaths.Parent = Leaderstats
	local Money = Instance.new("IntValue")
	Money.Name = "Money"
	Money.Parent = Leaderstats
	Leaderstats.Parent = Player
	
	Player.CharacterAdded:Connect(function(Character)
		Character:WaitForChild("Humanoid").Died:Connect(function()
			Deaths.Value = Deaths.Value + 1
		end)
	end)
end)

The code above creates the Leaderstats folder, Deaths value and Money value in the player which will be displayed on the PlayerList. When the Player dies, their Deaths Value is increased by 1.

Next, we'll need to attach some code to a Part so that when it's touched it'll convert the your Deaths into Money!

local Multiplier = 1
Part.Touched:Connect(function(Hit)
    local Player = game:GetService("Players"):GetPlayerFromCharacter(Hit.Parent)
    if Player then
		local Leaderstats = Player.leaderstats
		Leaderstats.Money.Value = Leaderstats.Money.Value + (Leaderstats.Deaths.Value * Multiplier)
		Leaderstats.Deaths.Value = 0
	end
end)

This code will convert your Deaths into Money. Right now you'll get the same amount of Money as your Deaths. You can increase the amount you get by per death by increasing the Multiplier.

Anything less than 1 will give you less Money, anything more than 1 will give you more Money.