If there is nothing wrong with the set, then it would be a change of mind return and I would refund purchase price only (upon receiving it back in the same condition as it was sent).

The best thing to do is work out a return policy and have all the terms in the listing so that buyers know exactly what to expect if they want to return an item, and you also have something written in clear detail to refer to in these kinds of situations. (My own policy is full refund plus return postage costs if an item is faulty or not as described, refund of purchase price only for change of mind, if items are returned in the exact same condition and return postage the responsibility of the buyer).