If you are familiar with Frontpage, you can spend an hour or so on a template, then copy the code onto the box on ebay. Frontpage is actually pretty easy to use, its like word, except more geared towards html. You just model the page to what you want it to look like, and it will convert it to code for your. Other tips are:
1.) Don't overbold. People will get numb to it. Use it to stress the importance of something, thats it.
2.) Make sure your ad has a structure that's easy to follow. For instance you dont want to have a page that has small print, then large, a picture, then more small print, etc. You want to make your ad have a good flow to it, from beginning to end.
3.) Structure your ad so that people can see the highlights at a glance. Much like an english paper so to speak. Paragraph for small intro into the product they are bidding on with some suggestions as to the use for your product (to givem ideas in case they are just window shopping). Bullet list style of the features, use 3 columns if there is alot, no one wants to scroll down for 15 mins to see the rest of your ad. Have a small concise detail of how the transaction will be handled, etc. you get the point :)
4.) I cannot stress this enough, grammar and spelling are very important, you dont want to appear unprofessional, always run a spellcheck.
5.) Finally, please proofread! It is truely amazing how many people fail to preview thier ad, then have to go and revise it later (me included!), and thats providing that they catch thier boo boo! Always think to yourself when reading your ad, 'What does this say to the reader about me as the seller, and my product? Does it make my product look good? Does it make me look like a professional?' Your product image is important for obvious reasons, and your personal image is very important because it will help the reader decide whether you seem like a trustworthy person or not, and that in itself can make or break the deal.
Hope this helps!