eBay's policy protects buyers over sellers which is fine but when you consider the fact that the buyer is a scammer, non-payer, etc and the seller is a trusted PowerSeller then the whole policy becomes flawed.
I have a PowerSeller account and a personal account and both were 100% positive until one member decided to leave a negative score after asking me for more items for free! Unfortunately eBay will do nothing about this unless the buyer has actually stated that he is going to leave negative feedback if he doesn't get what he wants. So here is an easy way you can extort honest sellers 'till your heart's content.
The buyer in question actually had comments left on his feedback (from business sellers. 100% feedback sellers, etc) that were negative and warning other members about him. Unfortunately, if he buys from you on a BuyItNow listing then he can do what he likes to your feedback and there is not much you can do to stop this. His actual feedback score is 100% - because you are not allowed to leave a negative or even a neutral score.
I am unable to warn you of who this buyer is as I will be breaching policy.