Here's the deal. If the buyer is legit, then a responsible seller should accept responsibility and make things right. This might involve refunding the buyer for the entire cost of the transaction, or a portion thereof.
However, if the item is in otherwise good condition, and the transaction itself is suspect, then eBay policy requires seller to refund purchase price, but also requires buyers to return the item at their expense. If the buyer is in USA and the seller is in Japan, then the cost to return the item will be prohibitive. Additionally, buyers need to get delivery confirmation for any item less than $200 value, and signature confirmation on any item valued at $200 or more. This return cost is quite high, often exceeding the value of the item itself.
Returning the item means that EVERYONE loses. The buyer since he will lose whatever it costs to return the item, and the seller since he will lose the sale, and also lose the original cost to ship it. However, in this case, the boombox appears to be in otherwise good condition. Since nikonfoo will likely will be required to refund the purchase price anyhow, he might as well get the boombox back. It has value even if just for parts.
Of course if the buyer is honest and it was not his fault, then sellers should not make them hurt for something not their fault. However, if the buyer has a history of scamming or taking advantage of honest sellers, then backing down and letting them slide enables and encourages them to continue scamming sellers. Unfortunately, eBay's system is set up to enable dishonest buyers so holding your ground is basically the only way to stand up to them.
As for feedback, again, don't worry. Patterns usually are apparent and if you have a history of honest behaviour with an occassional negative, then most eBayers are sophisticated enough to understand that some buyers leave feedback for extortion purposes or are unusually hard to please. The totality of the portfolio is what is important. Besides, many people don't know this but feedback is not final. You can always leave follow-up feedback to feedback-left or feedback-received to explain.