I switched back to an old account this summer, one with low feedback. I had two sellers cancel on me for being "under 100 feedback". And a third that tore into me for being "too new to understand eBay" when I'd enquired about an item (it's mispriced, and she's surprised she's listed it 4 times with no interest?)
So feedback number seems to be something that sellers use to determine whether to go through with a transaction (both were about $50-$75, not high-risk categories, either).
On the other hand, as a buyer I've asked people to leave feedback if a week or two passes and some have been offended I ask, others do give feedback.
But I sort of wish that neg and neutral feedback could be left for buyers as well.