A good auction listing will contain a clear picture of the item being sold. It will list an enthusiastic, proof-read description that is free of grammatical and spelling errors. DO NOT put a sob story in your description about how your ex done you wrong or how your dog needs an operation or how Aunt Sally's house is going into foreclosure. Just the facts about the item are all you need.
If you are selling clothing, ALWAYS list the measurements and the colors of the item. Always indicate the condition of the item. Is it New, With Tags? Is it used? Tell your buyers about the item as if you were describing it to someone who is blind.
A good listing will also list payment terms along with shipping and handling fees or use of the shipping calculator so the buyer can calculate shipping and handling. Do not gouge your customers on high handling fees to compensate for starting your auctions at dirt cheap prices. It is unsavory business practices like that that give the rest of us sellers a bad name.
I recommend listing a return policy as well, though it's not always necessary. Be fair in your return policy, but be firm as well. There are people who will walk all over you if you are wishy washy. It's the law of nature. You don't have to use HTML or eBay's listing designers at all. They are nice and add nice touches to your listing, but they are not necessary.
The 4 items I listed; picture, description, payment, and s/h are all you need to have a basic, concise, clear auction listing.