I don't think the negative feedback is ignored purely for the reasons that Seth speculates. More often than not the negative feedback comments on ebay reflect rants that come off as a little nutty, and people kind of tend to ignore screamers. Another factor points to the weakness of ebay's reputation system in that the positive feedbacks almost always outweigh, in volume, the negatives (if they don't the seller will surely get a new ID.) The positive feedbacks are almost always full of AAAA+++++ and lot's of exclamation points, which is just one more reason why I haven't used ebay in years.
I think the reason is classic cognitive dissonance. For unrelated reasons, you've already decided to buy. Now, the negative feedback needs to be ignored in order to validate your earlier hunch that you wanted to buy.