How in App Purchases will work?

You will have items for sell in Epic store, you can buy them , then in the game you will have a button that will consume your purchases and pass to inside the game the items.

Are there items (such as additional content, offers or currency) for sale that are used in the gameplay of your product?

Yes specially items for the game.

Can your product be used without making any in-app purchases?

Yes it can.

Are in-app purchases refundable?

No.

What payment platform is used to facilitate these purchases? Do you use Epic Games’ platform or a 3rd-party (including your own) payment system?

Epic Games platform.