Every eBay user has a feedback profile.  Your feedback rating consists of the number of positive feedbacks and comments you have received from past buyers and sellers.  The feedback system on eBay is vital to building a user's reputation.  You have 90 days from the end of the transaction to leave feedback.  After that time, the item is deleted from eBay's system.  Once feedback has been left, it cannot be erased.

After you have bought or sold an item on eBay, you should always leave feedback.  eBay's feedback system helps to determine a buyer or seller's credibility.  If you have purchased an item on eBay, you should leave feedback as soon as you receive the item.  If you are a seller, you should leave feedback as soon as you have received payment.  If a customer has paid you for your item, they have fulfilled their end of the transaction.  You should not get in the habit of waiting until a seller has left you feedback first.  As a seller, if you have described your item honestly and shipped your item when you said you would ship, you should expect the buyer to leave positive feedback for you. 

Often, a new eBayer will purchase or sell an item, not hear from the seller/receive the item and immediately leave negative feedback.  It's a good idea to attempt to contact the buyer or seller, BEFORE you leave negative feedback for them.  While there are some "deadbeat" users out there, often it is just a misunderstanding that can be cleared up.  Most problems can be solved by improving communication.  Leaving negative feedback on impulse, can damage a perfectly decent buyer or seller's reputation on eBay.

If you receive negative feedback for a transaction, you should still attempt to contact the user who left the feedback to clear up the issue.  Although feedback cannot be erased, a user can go in and add additional comments to explain the negative if the issue has been resolved.

Additional Resources:

Best Times to End an Auction

Starting Your Online Auctions Business

Book Review: Starting an eBay Business for Dummies