i need help with these three points

Your group has an electronics shop that sells both new and used devices. Some of the devices are used for demo purposes; after 6 months, these devices are then sold as used devices, eg TVs, cellphones etc. Should your group business record these sales of used devices as revenue or as a gain? A partner insists that a main indicator of If control has passed to the customer is if revenue has been earned. Is this a true statement? Why/whynot? A partner wants to know when you should consider fulfillment of the performance obligation and recognize revenue over a period of time instead of at a point in time. Explain the difference between them