A bit more information would have helped us to help you.
Buyers can't cancel purchases on eBay, only sellers can. If you buy an item by mistake you have to pay to return it, assuming that the seller accepts returns.
If the return postage is more than the item is worth, did you perhaps buy it from a seller in China? If so, where was the item located - this will be shown in the listing page. Many sellers in China have a distribution base in the UK. If the was located in the UK, the seller should enable you to return it to their UK address.
If they still try to insist on it being returned to China you may need to contact eBay customer support.
