I’ve just been on the other end of this a week or so ago
bought some brakes on the bay described as working fine, bled and ready to go, when they turned up the master cylinder wasn’t returning properly in either lever, might have just been bad bleed, might have been fubar’d, either way I didn’t want to go trying to fix them and run the risk of then being told I’d made them worse so requested refund and return.
I posted them back at my expense, seller refunded me original purchase price but not postage, despite me sending scan of receipt so going on that I think you don’t *have* to refund their postage, but you can get the fees back.
I probably should have asked the seller to refund my postage cost though, as its not right that the buyer should be out of pocket if the goods are not as described.
depends whether your buyer is trying it on or if there is something genuinely wrong that you missed.
I’d say request they return it recorded delivery at their expense, if it arrives and there is a genuine fault you should refund the buyer in full. If it turns out there was nothing wrong an they were trying it on then refund the sale price but not return postage as the item was not faulty.