are they trying to swap their broken one for your newer one?
do you have any way to confirm the item they send back is the one they sent you?
i know some people buy identical iitems to ones they already own, then return their broken one - they also do it by taking parts too - i dont sell much but i would always take all serial numbers, take many photos, note any scratches etc - and also make a tiny mark somewhere on it so that you know its yours.
otherwise, if they do have a case, you must pay the postage, if the item is faulty - they pay it if they have only changed their mind.