My current experience makes me support buyers not being able to leave negative feedback.
I just bought an item & paid for it the same day by PayPal (the only option). The seller didn't accept my payment immediately, but (unknown to me) relisted the item as a buy it now, for 25% more than I paid. It sold, he accepted the other payment and rejected mine. Needless to say he tells me it was all a mistake.
I've left negative feedback, but now I'm waiting for him to do the same in retaliation.
As Baz94 says, I had to think long and hard about whether to leave the feedback and run the risk of losing my 100% positive record.