Yes, the seller can only leave no feedback or positive feedback. Just another example of eBay's buyer-friendly and the-hell-with-the-seller approach. I wouldn't do this, but some sellers leave a positive feedback along with a negative, scathing remark as the feedback comment.

I'd email the buyer both by sending it to his user ID (eBay's internal email system) and to his regular email address (if it is available to you). You could USPS mail him as well. And use the Contact Info mentioned elsewhere in this thread. Using his eBay ID, I'd do a buyer and seller search on him to see if he is still active. He could be dead or ill.

Finally it is good to use the eBay complaint mechanism mentioned in this thread -- file an Unpaid Item Dispute with eBay to get your final value fees reversed if you end up unpaid; it might even shake up the buyer into paying. In the complaint and in separate emails, I'd let the buyer know that you are willing to forget the transaction if he is unhappy for any reason, that the auction could be cancelled, that you want him to be happy and are only interested in getting your final value fees reversed, and that if the auction is cancelled or he pays, all this complaint paperwork is forgotten by eBay once the complaint is mutually, satisfactorily resolved. (If the auction is mutually cancelled, you get your final value fees reversed.) (If the buyer gets a lot of unresolved Unpaid Item Disputes against him, he could be suspended, etc, by eBay.)

Robert