where I complain him
There is nothing to complain about. All returns and refunds should be made through eBay. Your buyer needs to open a case under eBay's money back guarantee. If he is claiming that his item arrived damaged you then send him a prepaid return label. When the item is returned to you eBay issues his refund.
You are selling multiple brand new items whislt still presenting yourself as a private seller. Private sellers may only sell their own, unwanted possessions. Buying new items to resell on eBay is a business activite and you need to be registered as a business seller.