I have 4,356 overall positive feedback ratings as of this writing. I'm not bragging, but using this as an example of how to EARN positive feedback on each and every transaction and sale you have on EBAY!

Selling: Be sure to list your item with a clear concise and honest appraisal of the item. Note any flaws in your description. Buyers appreciate honesty. If you make a mistake in your rating of your item and a buyer questions you on it, admit your error and try to offer a partial refund or store credit.  Buyers appreciate the effort to rectify the situation. Take pictures, post details.  State your return policy and rating policy very clearly. Set your listing up so that all of the important details are prominent.

Buying: Pay immediately using paypal or money order. If you send off your payment the next business day after your purchase, sellers appreciate and recognize the fast payment and your efforts. Don't be a deadbeat bidder/buyer. If you bid/buy, pay for the item regardless of whether you have "changed your mind" -- once you've bid you have a binding contract to purchase the item. 

Whether buying or selling, honesty is the best policy. Make the sincere effort to assist your buyer or seller. A positive rating almost always comes from the extra effort you give.