How do you script a way to buy catalog items in-game?

To buy an item in game, do the following:

local market = game:GetService("MarketplaceService")
local player = game.Players.LocalPlayer -- note, this has to be a LocalScript for LocalPlayer to work.
local itemId = 00000

market:PromptPurchase(player,itemId)

http://wiki.roblox.com/index.php?title=API:Class/MarketplaceService