There have already been a number of threads on these boards about exactly this scenario where a buyer is instructed to return an item for a refund and they simply return a tracked parcel with nothing in it.

Then ebay/paypal will refund because the tracking says delivered.

Very little the seller can do to protect themselves against this kinda scam.

Arguing with ebay/paypal gets you nowhere. They will simply go with the tracking

The only thing I can see to do in such a case would be to lodge a formal case with The ACORN and hopefully the federal police would investigate the buyer for fraud.

If ever we are in a position to accept a tracked return I would make sure I had another person present when I opened the package. Even video it being opened with the other person being in the video. Then if it was empty they can sign a sworn stat dec saying it was empty when opened. This then is taken as legal fact especially if you have accompanying video evidence.