Unfortunately you must pay the return postage because you neglected to include in the listing that the buyer is responsible for all return postage costs.
If it goes to a claim and the buyer wins, ebay will say to the buyer that they will pay for return postage by offering the buyer a means of posting it using a printed label to take to the post office. I don't know if ebay try to claim the postage costs from you but I suspect they would.
If it was me and a buyer tried it on by saying it was dirty like a 5 year old phone I would refuse to accept the return.