If it has been at least a week since the auction ended and you have not heard or made contact:

• If you are the seller of the item, you can contact the next-highest bidder to see if he/she is still interested in the item. After a week of trying to get in touch with the original high bidder, it's perfectly acceptable to move on, even if extraordinary circumstances have kept the bidder from responding.
• If you are the high bidder of the item, your obligation to purchase the item ends when the seller cannot be reached.

If you have tried email and the message center, you've done your best. In any case, you can use the feedback system to leave either neutral or negative feedback about the user, to help other users decide who to do business with.