yes they should pay - many think that because they write on the listing that they wont refund postage that they don't have to - but they do.
problem is most buyers do not challenge it and just accept this.
if they have sent you an item that is unfit for purpose or the wrong item or something then you should not be out of pocket by one single penny - the mistake is theirs, not yours
if the buyer has just changed their mind or chose the wrong size or something, then it is the buyer that must pay - if the seller will accept a refund on that basis
you should explain this to them and demand they cover ALL postage costs - open a dispute if you have to, and explain this to ebay too
too many sellers use this as a way to make money selling shoddy or broken goods 'by accident' - they know most people wont bother to return items as the postage would be too high, so they rid themselves of crap stock and still get the cash.