If the buyer opens a return for "doesn't fit" or "changed my mind" then they pay for the return postage label. Your refund still has to include the original postage cost.
As a private seller you don't have to accept returns for change of mind, though the buyer can still open those returns and you have the option to accept or decline.
As a private seller listing multiple brand new items you may find yourself on thin ground if you are actually selling/trading for profit and don't accept returns (that would be against the law / distance selling regulations). So I think I'd be sure on this occasion to make sure you allow the buyer to return the item at their cost, and I'd consider whether accepting returns on "buyer pays" basis would be the better option for you in future.