Many people who buy on eBay just look for the cheapest price and wonder why they are sent something which is less than they expected. Feedback is one of the most effective way of assessing a potential seller or buyer and as feedback on eBay is such a powerful tool feedback should not be issued lightly (either positively, negatively or neutrally)
As a powerseller feedback is incredidly important to our company as both as a buyer and a seller. It is very important that you never leave feedback until you are 100% happy with your item however it is important to be fair about feedback as sometimes things happen that are out of control of the seller such as postal delays and goods damaged in shipment.
If the person you have bought from has done everything that they can to resolve the matter to your satisfaction then they deserve positive feedback if the situation was out of their control however on the other hand if your seller ignores your communication or has a 'not my problem' attitude even if the goods do eventually turn up then they possibly deserve negative feedback as this warns other buyers to beware!
Neutral feedback on eBay is often misused as people think "well i didn't get anything special just what I paid for in the time promised" and leave neutral feedback instead of well deserved positive feedback. It is my humble opinion that Neutral feedback should only be used in a situation where things have gone awry but not so disasterously wrong that the seller or buyer deserves a negative mark against their name.
Negative feedback should never be used vindictively nor should it be used to threaten buyers or sellers and eBay has very stringent policies in this regard. Most specifically negative feedback should never be used because the buyer misunderstood the Auction details or blatantly disregarded items such as postage costs.
I remember early in our eBay days we had a number of compatible Inkjet cartridges that we were liquidating on behalf of another company on a consignment basis and as such we had them listed as a 99c auction with no reserve and a $10 handling and postage fee. A fellow reseller won the Auction and then wanted to haggle about the delivery cost however we refused to budge as our delivery costs were clearly stated and despite offering to allow the 'gentleman' (i use this word loosely) to pick up the item to avoid the delivery cost we were punished quite unfairly with negative feedback.
This is one of the few pieces of negative feedback that we have ever received and it irks me to this day as I felt that our reputation had been unfairly tarnished...however when recently reviewing the eBayer in question I noticed that he had very little positive feedback and considerable negative feedback. If someone you wish to buy or sell to has negative feedback it is always worth doing a little research as to the reason why and this has become increasingly easier thanks to eBays recent development of the feedback system.