You're trading illegally (by not being registered as a business seller), and consumer law states that a buyer can return an item to an illegally-trading seller for any reason at all, and in any condition at all, for up to 1 full year and 30 days, and the trader has to pay their return costs too. The law doesn't reward those who break it. So accept the return and refund in full on receipt, as you don't have a legal leg to stand on. (You should also read Ebay's 30-day Money Back Guarantee to learn what '30 days' refers to and what your customers are entitled to.)
Anyone who makes, grows, refurbishes, or buys in (new or used) items to sell, is a business and must be registered as such. Not only an Ebay requirement but the law. Note that there is no such thing as a 'hobby seller' in this country, and how much or how little you sell or make is totally irrelevant. Private accounts are ONLY for those selling off their old no-longer-wanted possessions from wardrobe and attic. That clearly isn't you. For confirmation, go to Ebay's Seller Centre (link at foot of any page). For full information, go to the government's website.
I would advise you to upgrade your account from private to business (takes seconds) as a matter of urgency, before you find yourself in big trouble with Ebay and Trading Standards. Most buyers are savvy these days, and your particular buyer may be one of them and may already have reported you.
@lethal_prices
