How can I return it?
I do wish eBay would ask people with questions to provide at least basic information.
How can we answer your question without knowing anything about the sale?
When did you buy this phone, and how did you pay? What is the reason for returning it? Is your seller a private seller, or a business? In the UK, or abroad? What is their returns policy? (This will be statedin the listing.) Did you buy it through your eBay account, or as a guest user?
The procedure you need to follow and any rights you may have will depend upon this information.