This is always a worry - I bought two items in the past year and found the sellers had gone unregistered after I had paid.
Much to my surprise, both sent the goods! The second only after an inordinate delay and a few reminders from me.
In future I will adopt what I am advising you here - at least you will know sooner. Apply to eBay for their full address and then look them up in the phone book. I have found that ringing people can make them behave better - it is more difficult to defraud when you have to explain your actions.
In the end if you have paid by Paypal, contact them and ask for a refund. I have only done so once and they delivered.
Paul