There is a facet to negative feedback that is even more important than making sure you leave it  fairly and honestly.  The crucial piece to the negative feedback puzzle that many people miss is that negative DOES NOT equal nasty.  This is important, because as both an eBay buyer and seller, I can honestly say that I have not bid on the merchandise of sellers who leave unprofessional feedback.  There is a right and wrong way to leave feedback.  Here is an example:

A buyer wins and auction and you are unable to make contact with them for three weeks.  When you finally do get a hold of them, they say they've changed their mind and won't be paying for the item.  Here are some hypothetical negative feedback reactions (we'll leave disputes out of it for this example):

"Buyer SUX.  Total run-around.  Don't do business - total loser.  F---------"
or
"Buyer difficult to contact, withdrew bid after auction ended."

Or, let's say a seller sends you something you feel (and perhaps rightfully so) was not as it was described in the auction and then refuses to offer a refund:

"F-ing LIAR.  Seller total $^%*, screwed me out of $$$!!!"
or
"Item not as described - seller refused refund"

Now, in either case, negative feedback is being left, which can be seen by potential buyers and sellers.  Any of the above feedback will result in a point of negativity that affects the eBayer's record.  Both the emotionally charged, slang-ridden responses and the professional, objective feedback give other eBayer's the same information about the person you left feedback for.  The difference is, when you leave nasty, vindictive, unpleasant feedback, it's a reflection of YOU, not the person you're trying to hurt for swindling you. 

You might ask, "Why would you be afraid to do business with a seller who leaves nasty negatives unless you were a deadbeat buyer yourself?"  Good question.  The answer is, life happens.  If I wind up with my child in the hospital, or get called into work, or have to face a snowstorm and can't get my payment out for a day or two, I don't want to risk the outlandishly rageful, ignorant-sounding feedback from said buyer.  Of course, as a responsible buyer, I'd email the seller, but that doesn't prevent them from leaving you a negative. 

Finally, don't leave negative feedback because you messed up, got a much-deserved negative, and you feel it's PAYBACK TIME!  If you deserve the negative, take it and leave a follow-up reply (politely) if necessary.  Do not turn around and leave a retaliatory negative just because you're angry that your feedback reputation has been tarnished.  Be bigger than that, because if other buyers are like me, that too will lose you bids every time.

The point is, say what you have to say, but say it so you don't sound like a would-be Jerry Springer guest.  The eBay community at large will respect you for your honesty and tact.