Selling an Item
If you have Builders Club, you can create and sell some of your creations (except Models, Decals, Places and Audio, which are either free or private). If you want to sell an item, a T-shirt for instance, you would go to the item's page, click the small gear icon above the green 'Buy Now' button, followed by the 'Configure this T-Shirt' link.
Once you are on the configuration page, scroll down until you find the section titled 'Sell this item' and tick the 'Sell Item' box. There are two ways to sell items: you can sell them for Robux or Tickets, or both! Now assuming you still have your Builders Club membership you can receive the money every time someone buys your item!
Keep in mind that 30% of your profits of every item sold will be given to ROBLOX.