Ebay's feedback system is an excellent way to gauge whether a seller (or buyer) is trustworthy and reliable. This is exacly why you should do all you can to Keep Your Feedback Positive! Inevitably, you will probably receive a negative feedback at some point. It's purely a numbers game.
Don't let this worry you though. There are a few things you can do to reduce the chance of receiving negative feedback.
As a buyer, you should always leave honest feedback once your purchased item has arrived. BUT, never leave a negative comment before contacting the seller to resolve any issues you may have. It can be tempting to just launch into a rant if there is a problem I know, but you have to remember that it may not be the sellers fault. Almost all will be willing to help as far as they can (and refund if needed).
If you leave a negative comment, nine times out of ten you will receive the same in return, so think it through thoroughly before you decide!
As a seller, it is advisable to wait until you have received feedback from your buyer before leaving it for them. This way you can try to avoid those few obnoxious users who revel in the chance to leave a scathing negative feedback (same reason some people buy big cars maybe?). Ask yourself this; How many users are afraid of reciprocal negatives? Most of the time the random negative leavers will only do it if they no there's no chance of getting one back.
The best way to avoid negatives as a seller is to keep buyers informed all through the transaction. If there's a delay at all, let them know. Follow up a few days after shipping to see if they've received and have any queries. Good customer service is the key!
Going back to my earlier comment, you inevitably will receive a negative feedback at some point. Don't panic though, you can deaden the impact to almost nothing very easily. Simply use the 'reply to feedback' function and tell your side of the story! You can also 'follow up feedback' for the user who has left it for you if needs be.
A question for you; Do you care whether a seller has a 98% feedback rating as opposed to a 100% rating? I thought not, and other buyers don't either!
Don't fret if you do receive negatives. Yes, an occasional negative rating will drop you below 100%, but if you can reassure users browsing your feedback, it's really nothing to worry about. Remember, if you do feel it's affecting your reputation, there's nothing to stop you creating a new eBay account and starting afresh.
A final tip; If you do receive a negative, calmly contact the user who left it. A lot of the time you can work the problems out and agree with them to go through the 'mutual cancellation' process.
So there you have it. Do all you can to keep your feedback positive, but don't panic if you do receive a negative. IT'S NOT THE END OF THE WORLD!