Make sure you’ve downloaded ROBLOX Studio ... Uploading and/or selling items on ROBLOX incurs fees for the player. Fees are paid using ROBLOX’s premium currency, Robux, which you can buy ...

Remember, the Roblox demographic is mostly made up of young children and teenagers. So, if you charge Robux for a perishable item, make sure that it's also available without purchase in some fashion.