It seems you posted them all in one parcel and the buyer lost his claim about not receiving them because tracking showed as delivered. Then the buyer opened a return case.

Probably you may have been better to accept a return, even at your expense where you sent a postage label.

That way you may have received your cards back.

I say may, because the buyer could basically send any cards back and it would be hard to prove they scammed you.

But if ebay is now refunding on the basis of items not received, maybe contact chat and remind them you have the tracking record and that is why the claim was rejected first time. Are you sure it was not an item not as described claim the second time?

My only suggestion for the future would be to sell cards individually, maybe with postage cost included in the cost so you can label it free postage.

That way, when you sell, you could post each item individually. None of this bundling into one parcel.

By having 'free' postage, you'd save yourself from having buyers ask for combined postage, which in buyer speak always means-Can you post it cheaper?

Sending cards individually wouldn't necessarily stop a scammer but it might make it harder. It's easy for them to claim you made a mistake when a bundle of cards has been sent, harder if they are trying to open eg 6 different claims against you.

In the meantime, block that buyer from ever buying from you again. Sounds like a right scammer.