Piffle.
Tell him to ask you for a Mutual Cancellation.
http://resolutioncentre.ebay.ca/
If you agree, the transaction ends amicably and there is no comeback on either of you.
When he sends the request, agree immediately.
You both will be able to leave appropriate feedback and Detailed Seller Ratings. Hmm. He may be nervous of your reaction to the problem. Assure him that you understand that stuff happens and that you would appreciate hearing from him when the item is back in stock.
If you think he is some sort of scammer, you would react differently and I would give different advice. Since you have not paid, I think this was just a stock problem.