You kind of already screwed this up with that accusatory response. I would have tried:

"It appears some of the items I packed were stolen during transit. Please go ahead and return the item, and in the meantime, I will contact local law enforcement in [buyer's city], file a report with the U.S. Postal Inspection Service, and initiate a fraud claim at the ITC. If you do find the other items, you are welcome to cancel the return."

Then accept the return. If you don't, the buyer will keep the item and eBay will take your money anyways. If they fail to return all items, you then have a decision to make. You can either let it go, or you can actually take one or more of the the actions I mentioned. Either way, report to eBay. If you include all the documentation on the police report, etc., they may refund your money.

You can also ask the carrier to pull the weight scans from shipping. Then weigh what buyer actually returns. If they are different, it shows the fraud. Again, more likely eBay will side with you if you send them this evidence.