There is alot of discussion on this topic through the boards here, so I guess thats why responses are low on this topic.  But I'm new to posting here so I'll add my 2 pennies worth. 

I guess one of the reasons is because the last thing you want as a seller is to discourage buyers from shopping on eBay.  If sellers could give negatives I think the whole dynamics of the place would change, thats just my opinion. In the end all the buyer has to do is pay for the item in the time specified by the seller or there abouts.  If they dont pay you can always cancel the transaction and relist the item or if there were other bidders offer a second chance to the next highest bidder.  Essentially it's the buyer who is taking the bigger risk IMO they are the ones who have to pay for something before they have it in their hands. There are so many "chances" (for want of a better word) that things can go wrong ie; they have to trust that the seller is legit and honest in their description, that they posted it when they said they did, that the postal service does their job properly ect ect and this is all before the buyer even gets to see their item in the flesh (so to speak) 

On the other hand some sellers probably deserve negative feedback, but I guess these are the ones that you make a point to "block" so that you will never have to deal with them again.