It may be tough if the seller cannot be reached. Another thing to think about, do you really want to send money to someone who has suddenly stopped responding to you? How responsive to you think they will be should the model arrived damaged or in a condition other than they stated if they are not responsive BEFORE they get your money?

Try emailing him one more time and tell him that if he doesn't respond by (insert your own date here), you will consider the deal void and will move on to something else.