There’s a slight trap in this- if you combine price and postage into a single lump sum and call it “free postage” then that’s exactly what ebay count it as- so if you make a refund, they normally expect you to refund the full amount, since the postage was “free”. (I get bitten by this every so often, it’s just a price you pay for hte simplicity and advantages of “free postage”)
But I can’t see why you couldn’t agree up front with the buyer that as an exceptional thing, you’ll accept a return without accepting any fault, and you’ll refund the price less the original postage cost, and also require them to pay the return postage. So essentially refunding so that you are not out of pocket for their change of mind. You do have the option of making a partial refund. It could get messy if the buyer’s an arse.
Having said all that, if the buyer’s coming back to you politely and requesting a return, you can probably assume that they’re not an arse- because if they were an arse they’d just abuse “not as described” or break it and return it as “damaged”.