You can use stamps to buy Special Delivery postage over the PO counter. That's fully tracked.

But I meant in cases where the buyer has received the item and hopefully is happy with it. They may not be expecting the postage they paid (for an unused SD label) to be refunded to them.

Under these circumstances isn't it just possible that Ebay may be tempted to keep the money?

We'd need someone (not me) to buy something AND for the seller to use alternative postage to find out... 😉