How can I sell items in-game like Shirts,Pants, and T-shirts?

There is actually a specific service for things like this called MarketplaceService.

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

The specific method you want here is PromptPurchase(Player, AssetId).

Example code:

local function RequestPurchase(Player, AssetId)
game:GetService("MarketplaceService"):PromptPurchase(Player, AssetId)
end