Try offering buyers an alternative and ask then if that would suit or would they prefer it if you cancelled the order. ie word it so they ask to canecl the order and then you can select that option rather than go direct to cancel order option.

At the end of the day as far as Ebay is concerned better sellers either send out damaged stock/wrong stock/empty box/"loose" it in the post and fix it later IF buyer complains. Bad sellers are honest and tell buyer upfront they are out of stock or damaged. Go figure..