I was at 50% for awhile, because when I was new to eBay, the second item I purchased, the seller hadn't responded to any of my e-mails and I hadn't received anything for a few weeks. I left a negative feedback and the asshat in return left me one, when I payed right after the auction ended. I eventually got it a few more weeks later.
Even now, I'm only at 97.3% (which is low by some standards) because of that one and only neg. The rest of mine are positive, but I only have 30-something transactions so I haven't gotten a chance to raise it.
eBay needs to do something to prevent retaliatory feedback. I don't think feedback left by member A should be visible on member B's profile until member B in turn leaves feedback for member A. That would help somewhat.