Can I sell accounts on Fiverr?

For eg Xbox / Fortnite etc?

No.

No. The platform does not matter. The sale, even the mere passing on of access data for such accounts violates the terms and conditions of the services. With the mere attempt to share with friends, the operator would already be authorized to permanently block the account.

You only have one right of use. This is not transferable and consequently can't be traded at all. The account is and remains the property of the operator.