The first thing I'll say here, which I will proabably repeat several times over is: Customer Service.
Be friendly, not too friendly.
Reply quickly. Performance is king. That buyer could have asked ten sellers that same question. If you are there, reply - chances are they'll favour the seller who replies quickest in the best manner. Customer Service rules.
Address every single concern. Answer all questions they have - even if you don't want to be honest! Always pays to be honest - remember honesty stops over 80% of returns. Customer Service Rules.
Returns policy - have one, or get one. The moment you buy to resell, you have to adhere to the law. Check DSR's out on the web (distance selling regulations). If you wont pay return postage costs on items - state it, otherwise you are liable to pay them.
Item descriptions. Ensure they are thorough. Try and describe every item as carefully and concisely as possible. Mistakes lead to unhappy customers. Declarations of no damage when there is some, lead to returns, and unhappy customers = negative feedback. A good seller, who describes their items accurately will have a high return buyer rating (where the product warrants it!).
Postage Costs. State the postage cost. Include the options available to each buyer (IE: UK, Europe, everywhere else). Advise via what method you send items. Take time to check out Royal Mail policies, and ADHERE to them!!
Packaging Items. ALWAYS parcel carefully. It IS your responsibility to get that item to the buyer in the same condition it left you. Remember this, if they receive broken - they'll complain, sometimes will leave negative with no warning. This is something you can stop before it happens! Especially to ceramics/glass sellers. Use that extra sheet of bubblewrap, or another layer of peanuts - they can save you an angry customer, and a claim to Royal Mail.(YES! Royal Mail DO compensate for breakages! I'm living proof!)
Too much tape! Every buyers nightmare. Do not overtape. I can't stress this enough. Use only what you need to secure the parcel. Royal Mail advise that taping around the parcel "H" shaped - is enough to ensure parcel stability. Can you imagine having to cut through a box of PURE TAPE? Believe me, its the most annoying thing!
Payment Options. Be clear about what you accept. Be clear what you don't! Always state this in a clear fashion, without being too forceful! Buyers are frightened away easily.
Prices. Keep them competitive. See what others are selling and how much for. If you have a shop, lead buyers to it by dropping the price on a buy it now by a few pence in comparison to the same auction. Penny-saving individuals are out there, and it is those buyers who generate more shop business.
Just be approachable. Think about every question you receive from a buyers perspective. Don't laugh at, be abusive to, or ignore your buyers. They're your bread and butter. Remember overseas buyers who ask what may seem like daft questions, may have been previously burnt by a less than honest eBayer. Lets change people's perception of eBay - give them a good service. If they don't return, then they might praise eBay to their next door neighbour, who might join eBay and buy from us as a result.