I got caught up in one of these scams a few months ago. Ordered some used Wharfedale speakers from China and they sent a USPS tracking number. A couple of weeks later I was notified that the item had been delivered - but of course it had not. I filed a claim with ebay and at first they were reluctant, and they even told me that the seller was claiming that I was scamming him!
So I got on the phone with USPS and they explained how sellers like this generate a tracking number, then that number is "highjacked" and used to send some unrelated (but probably illegal) item. In my case it was an "envelope" sent from a warehouse in Tennessee to an address in my zip code (but not mine).
I documented all this and sent it to ebay, after which they refunded my money. So it worked out OK, but I had to do a lot of work to protect my interests.