Well I've been on the other side of it. I ordered some car parts from a fairly big seller with around 99% feedback. He couldn't offer registered post even when I specifically asked him, and in the end the items never arrived. I opened a dispute and they basically said "sort it out amongst yourselves", even though he didn't have any proof of postage and started ignoring mails from both myself and eBay. I couldn't leave him negative feedback as I knew he'd leave me negative feedback, and it would be far more important to me with a low number an 100% than for him with his thousands...
In the end I had left it too long to reclaim the cost through PayPal, but even if I did, and even if the dispute had been awarded to me, I'm sure he would've left me negative once PayPal reclaimed the money. It's a harsh world really, and I know there are many, many genuine sellers out there, but I think neither feedback system works. As it was, the seller could withhold feedback, even after the buyer had done their bit, giving them unfair power over small-time buyers, but now it's the opposite way around. At the end of the day I've still got 100% for what it's worth, but took a hit of around €100 to keep it...
