What NOT to do in the Description
Don't make a one line posting: Really want to decrease your sales. Write a dinky one line description like this:

It is just not good salesmanship. You want to use your description to enlighten the buyer. This description gives NO details about the condition, model, etc... of the items. There is no human element. It is also sloppy and gives me the impression that the item will be sloppy too. Don't follow this example.
Don't be tacky: This seller decided to use HTML to jazz up his description, but his choice of fonts, size, and color were so tacky that it certainly detracts from the appeal of the item he is selling. It is hard to read, distracting, and just generally bad. Check it out:

And if you scroll down from there, it gets worse:

It's just like I talked about earlier about listing a long and annoying list of specs. There is no story, no feeling, no human element, and therefore no credibility. I hope you can see from this example what NOT to do.
Don't give your phone number or personal email address in the description: You want the readers of your post to be able to contact you easily, but putting your phone number and personal email directly in your description is risky. By doing this, your phone number/email is stored on a page on the servers at Craigslist. This page is public and able to be accessed by Internet surfers and the general public. Spammers/scammers often have programs called “spiders” scrub the Internet for phone numbers and email address to add to their lists. If these pages are freely available, you don't want your phone number/email posted because you run a chance of being found by the spiders and added to a spam list.
Instead use the email that Craigslist provides you in your posting which we talked about earlier. It all ends up in the same Inbox anyway and is perfectly sufficient for our purposes.
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The above should cover just about every aspect about writing a good description. Now onto what you should do after you write and submit your posting.