Your item description is crucial to the success of your listing.  Good descriptive writing is relatively easy if you follow a few guidelines, and will make the difference in the possibility of sale and the final bid amount. 

There are rules, there are always rules.  The fun part comes later.

Rule #1  Don't lie.  Don't falsify, invent or mislead.  It is a well known fact that Moses dropped one tablet on the way down the mountain and the commandment about listing counterfeit stuff as authentic or claiming a copy is original or any other such deceptive practice was lost.  It is still, however, very much a sin.  If you break this commandment, you will be lost in that netherworld of shame known as bad feedback.  Your karma will be the subject of jokes in neighborhood bars and dogs will slink away baring their teeth, when you approach.  This rule is very simple - do not lie.

Rule #2  State the facts.  Size, color, age, fabric, any hard facts should be included and stated clearly and concisely.  You can weave them into a narrative or just list them neatly in a row.  The more the merrier.

Rule #3  Condition of used items is a highly subjective matter.  What is mint to one person is fair to another.  If there are flaws, say so.  It helps to tell the buyer how to minimize or correct the flaws (dry cleaning, a quick touch-up with shoe polish, whatever) but don't just ignore the issue.  If something has been worn once, it's used, period - it is not NWOT even if it looks as if it is.  If this was your personal property and you've only worn/used it once or twice, say so. Your honesty will pay off with happy buyers. 

Rule #4  Do not, ever, use the term...I can't even type it.  It's the letter immediately preceding M, followed by the letter right after N, then with another like the first one.  Add ly and you have the beginning of the word lollygagging, but in this case it's just gagging.  Use any other way you wish to make your point, but do not use that.  It won't affect your karma but it will affect your credibility. 

Now wasn't that easy?  Now the part where you can play and perform  linguistic feats of daring comes into the picture.  That's what your description should be doing, you know, drawing a mental picture to go along with those nice little digital ones you've provided. 

Does the item have a past that you actually know about?  Say your dear old Aunt Phoebe, who was something of a lunatic and took to drink in later years, posed for art classes in her twenties.  You inherit her huge  collection of mens cuff links.  As a child, you could never figure out why she had so many of them, and when you asked, she said they were gifts, but your mother looked really angry and your father just turned red and looked like he was trying not to laugh.  So, here you are with 173 pairs of cuff links.  Do you write "Vintage cuff links from my late aunt's estate.  Many are solid 14K, some with jewels.  Quite a few are engraved with initials.  I am selling these as-is and if the buyer is approached by the minions of the law, demanding their return to the rightful owner, I cannot be held responsible.  The old woman was a drunk but that isn't my fault".  Of course not!  Shame on you to even think of saying such a thing!  No, you would write "Vintage cuff links from the estate of my late aunt.  Aunt Phoebe was a free spirit - a model in her younger years, she later became entranced by the oddest little things, which she collected avidly.  She never said so, but I think she was trying to get the entire alphabet in initials on cuff links, because she has every single letter at least twice, except for X and Z, which is probably why she just kept trying.  She was quite a character, always in high spirits.  A small piece of her lifelong passions are now available for you, the lucky winner of this auction." 
Isn't that so much nicer?  You didn't lie.  You gave the description a bit of interest and color while remaining true to the facts.  This works if you know exactly where the item is from and who owned it.

Otherwise, whether the item is used or new, you want to give the reader reasons to want this item.  Look at it.  Think of  uses other than the obvious.  Suggest places and circumstances it could be used or worn.  Tell them why you like it.  If all else fails, pretend you are in an actual shop, in front of a real customer, and see yourself selling this item - or getting fired for poor customer relations.  When the origin and history is unknown, it is at the same time easier and more difficult to write the description.  Easier because there is no real history you need to work in and around, more difficult because you're going to need to use your imagination and powers of suggestion. 

Details are important, and a bit more flexible than the size, color, condition facts.  Details can be something obvious, like a seriously ugly mix of patterns, that is apparent in the digital pictures but not something you want to dwell upon in your listing.  In that case say nothing about it. It could also be something easily overlooked but wonderful - a truly beautiful tiny button, the fact that all seams are edged with lace, a patina that was hard to capture on film - any of these would be worth mentioning and should be included. 

Last... try not to rely upon spell check, but do make sure every word is spelled correctly and used in the right context.  I can't count how many times I've seen patent leather listed as patten, or sequins as sequence.  I'm waiting for something with a sequenced patten, at which time I will get out my copy of Eats Shoots and Leaves and wave it on street corners, ranting hysterically about computer spell checks and grammar checks and the type of people who trust them.  They will almost never tell you when a word is out of context, as in sequins and sequence.  The computer does not know, and surely does not care, about sequins.  They are all familiar with a sequence, but that's not going to help you when a buyer reads your listing and, thinking you're illiterate, passes by because they worry about the quality of an item offered by such a person.