That is not good. Sorry to hear that. But that happens especially if the bear was damaged.
If the bear was not dirty you can fight for it. But usually customer wins because of ebay protection. And it will damage your feedback.
Tell the customer that you put photos on ebay and there is not dirty spots can be found. But if the customer want to cancel/return the item and request for refund, the return shipping fee is cutomer's responsibility. Once the customer send the bear to you and and you receive the bear, you can refund the money to the customer, but only for the bear, not the shipping fee. That is the right process.
Maybe the reason the customer wants to return is becaue the pink bear tush (leg) tag is usually balack/white. But your bear is color tag. If you want to resell the bear, cut off the leg tag because it looks like fake.