This isn't rocket science, but it's amazing how many people just don't have a clue as to how to be a successful ebay seller. Well, here are all the secrets you'll ever need to know:
1) Sell what you know. Are you into old records? Sell those. Did you collect beer signs as a kid and do you know more about them than the average person? Sell those. Love to play video games? Sell those. In other words, use the knowledge you've collected from your hobby to help you become an expert seller.
2) Be cool. That's right, just be a cool person. If someone is unhappy with their purchase, refund their money. More than once I've had an unhappy customer and I've not only refunded their money, I've offered to let them keep the product, no questions asked. And more than once that person has come back to my auctions.
3) Be honest in your listings. Don't misrepresent your item. Don't lie, cheat, or try to trick anyone.
4) Ship quickly. As soon as payment is received, get the item in the mail. No excuses.
Pretty much that's it. Not too hard, is it? Now get busy already, would you....