I too have had the occasional experience with this type of ebay feedback 'blackmail'.
One overseas company I dealt with had a very high feedback rating, however on closer inspection, if you cross-referenced any negative (or even neutral) feedback that had been left for them, they ALWAYS left highly defamatory and damaging comments in return.
When the transaction I was involved with went bad (the item either never sent, or lost in the post) I was sent a threatening email by the company warning me not to leave any negative feedback, or else....!.
If many people succumb to these types of tactics, it will make the use of feedback as a means of determining a companies trustworthiness and credibility, worse than useless.
It's a major flaw in ebay's system.