eBay has always been a delicate balance when it comes to leaving feedback. As a buyer, you should objectively examine your purchase AS SOON AS you receive it and make sure it matches the description. If what you bought is not what you expected, but it matches the description, tough noodles. If what you bought is not what you expected, and does not match the description? Work with the seller to see if they're willing to solve the problem peacefully. If not, it's negative feedback and PayPal report time. This, however, is the ideal procedure that NEVER happens.
As it is, even if buyers think you shipped too slow, you get negative feedback. If they're upset that day? Negative. The box is wet? Negative. Their preferred candidate lost the primaries? Negative feedback. There are few buyers who are truly objective and honest, and eBay/PayPal already sides 100% with the buyer in the case of a dispute. You, as a seller, have to solidly prove that the buyer is in the wrong or you lose everything.
As a seller leaving feedback, the ideal procedure is to leave feedback as soon as they pay. Technically, that's when the buyer's responsibility ends. Sadly, this is never, ever the case. As a seller, I was told to wait until the buyer leaves feedback before I did. That way, would could retaliate against stupid negatives. Our official stance was "we do not consider the transaction complete until you are satisfied with the product. We will do anything reasonable to work with you if you don't like what you got." If they weren't reasonable and they leave a negative? We left one, too. It's as simple as that, and it works just as well as it sounds.
Sadly, eBay is going to ban sellers from leaving negative feedback. They have just left the door wide open for wanton negative feedbacks from buyers who KNOW they can't be retaliated against. "The mailman was 20 minutes late... It's negative time!" Bah! As someone who was involved in selling on eBay for over a year, I can safely say that I will never be involved with selling something on eBay ever again, and I hope other sellers band together and tell eBay what the driving force behind their success thinks!
Come on, eBay. Everyone keeps telling you how you should do it, but you keep doing everything backwards! Make it so nobody can see feedback left for them until both buyer AND seller have left feedback. This will essentially stop people from waiting until the other party leaves feedback, so you can leave what they left. It won't stop people from being idiots, but it'll stop retaliation. Make a warning page (a big one, because people are blind dimwit morons) that bugs the person leaving negative feedback that they should communicate with the other party to try to resolve the situation. Maybe even force an email, but that probably won't stop the crazies out there.
But nooo... You stopped retaliation by limiting the folks that make eBay work. Smart.