I am a seller and our policy is buyer pay for the return cost, but one of our item has defects, so we would like to pay the return shipping cost to the buyer. Can someone tell me how can I do that on Ebay?

Yes, you are required to pay return shipping for items that are not as described. Ask your buyer to open a return request next to the transaction in his Purchase History. Once the return is approved, he'll be provided with a prepaid return shipping label, paid for from your account.