Thanks to JuneAtNoon for today's topic idea!
The styles that sellers on Etsy use to describe their items are as varied as the items themselves. I've always thought that less is more with descriptions-- You want to cover all the important information, but don't babble or tell an unnecessary story.
What IS the important information, however?
1. What are you selling?
Be specific: Not "a shirt", but "A hand embroidered yellow cotton button-up shirt"
2. How was it made?
It isn't necessary to provide a detailed tutorial by any means, but state the technique used. Hand embroidery is different from machine embroidery or screenprinting, so it's important to say your items are hand embroidered!
3. What does it look like?
Assume that the buyer's monitor may not show the same colors or textures that your monitor shows. If the shirt is canary yellow, say so! Describe the artwork if applicable.
4. What size is it?
Provide measurements in addition to any common size descriptors (small, large, etc.).
5. What is it made out of, and how does the buyer need to care for it?
Specify the fabric content when applicable. Even if care seems obvious, at least mention it. For example, many people wear their jewelry EVERYWHERE so they may need to be reminded not to wear a bracelet with a fabric base in the shower or pool.
Above and beyond this basic information, you will probably want to include something about what the intended purpose of the item is. For example, I include a short paragraph about giving my monogrammed handkerchiefs as wedding gifts. If your item is made to order, it's smart to include an estimated turnaround time. You may also want to explain the reason for a seemingly high shipping cost (if the item requires Priority mail, for example).
Now, I'm off to check my own descriptions!