They have already damaged you now by leaving the neg feedback. Which I doubt ebay will remove.

So now I would do nothing more until / if they open a case against you.

Then you will have to refund as you have no tracking. So no choice.

And if you resolve the case straight away before it is escalated, by refunding voluntarily you will not get a defect under the new system.

But doing it this way it forces the buyer to make an official record on their buying account.

So I would sit tight for the moment. The buyer may do nothing more as if they are serial scammers they will not wish to make it official.