In relation to the buyer not responding - I see this as NOT BEING A CLEAR CUT SITUATION. The buyer may not be responding due to circumstances beyond their control. What if they are having computer issues, and have no other way of contacting you? This would explain the lack of responses. If feedback is left, as eBay recommended, I would keep if factual, and at a NEUTRAL level - "Contacted buyer. Allowed 30 days. No response. What happened?", would be a suggestion. Given that the buyer has 0 feedback, it is also quite possible that they just do not want to go through with the transaction anymore. The things is, you do not know what the story is! Perhaps, if you took steps to find out. Such as, if the seller is local, contact eBay for a telephone number and ring the buyer. Also, in the age of electronic mail people forget about good old snail mail. Perhaps send them a letter...good luck...
I myself have had negative feedback left on my account, where the issue was TOTALLY out of my control. I offered registered mail, which was refused. The item was sent. Guess what, the item "did not arrive". No contact from the buyer, no emails to try to work out whatever issues they had - just feedback left. How do they know the item was not sent? How do I know the item was not recieved? Why did I recieve no emails from the buyer to work out the situation? Did the buyer actually try to contact me, with me not recieving the emails? It is actually quite frustrating being an honest person and willing to help people where possible, AND REASONABLE, and not to be given the chance to do so!
With this said, when feedback is left, KEEP IT FACTUAL. To deal with someone who has 100% positive feedback says that they are perfect! The reality is, nobody is perfect. To me, a feedback of less than 100% reflects this. People make mistakes, people aren't strict enough, people are dishonest, people are lazy, there are a multitude of reasons why a transaction isn't 100% succesful - also some people ARE just plain scammers and/or jerks!
Think about it...