Feedback is broken down into three ways. You have buyer feedback, seller feedback and then the combined feedback score, which takes both into consideration. If you're a seller looking to build your feedback score, then I would suggest looking around the house for things you can sell off cheap. The cheaper you can afford to sell something, the better your chances are of building really good positive feedback.
Some people will tell you to buy lots of dollar type items to build their score. Of course that builds your combined feedback score and your feedback as a buyer. Now that might work in some cases where buyers only check as far as their nose and look at your combined feedback score, but if someone is drilling down on you feedback as a seller, then it will serve no purpose at all!