As a woodwind musician hopefully some of this will help non-musical sellers out when listing instruments.
There are some important details that buyers are interested in and it will save sellers a lot of time answering questions and promote more interest in the listing if these details are included.
Title of listing. Simply saying "Clarinet" or "Saxophone" isn't going to grab attention. Brand, model & key should be minimum inclusions. Also check your spelling! Correct spelling will help your listing turn up when buyers are searching. PS: It's "saxophone" with an "o" not "saxaphone" with an "a."
Brand. Which company manufactured the instrument.
Model. Nearly all reputable manufacturers will have a range of instruments from "student" to "pro" and buyers will want to know this. To use cars as an example, you wouldn't buy something listed as a Vauxhall without first knowing whether it was a Corsa or a Vectra would you?
Key. The pitch of the instrument. For Clarinets this is most likely Bb (B-flat). The next most common key is "A" but they also exist in Eb & other keys too. If in doubt measure the instrument (without the mouthpiece) from the top of the barrel to the bottom of the bell - a clarinettist will be able to figure it out from this information. Saxophones also come in a variety of flavours! Bb Soprano, Eb Alto & Bb Tenor are the most common but there are many more less common ones (soprillo, sopranino, baritone, bass, contrabass, subcontrabass etc...) both smaller & larger. If in doubt find someone who knows or include plenty of pictures in your listing.
Material. Especially relevant to Clarinets & Oboes, is the instrument made of plastic or wood? Some clarinets are wooden bodied but the bell is made of plastic.
Damage & overall condition. If it's a wooden instrument, check it with a fine tooth-combe for hair-line cracks and splits. List or photograph all cracks, "dings" (in metal instruments) & scuffs (no matter how minor) and particularly look out for ground off serial numbers - often indicating a stolen instrument. The lacquer finish on Saxophones has no bearing on their "playability" (comments such as "black saxophones have a darker tone" are hog-wash!!) but some buyers may want a shiny one whereas some people aren't bothered. Mention any lacquer defects anyway.
Keys - condition & "system." See the above comment about lacquer which also applies to the plating on Clarinet & Oboe keys. Most relevant to Clarinets, there are several different key "systems" the most modern (and standard) being the "Boehm" system but there are others too, like the "Oehler" system (the usual tell-tale is a roller between two of the keys - similar to a saxophone). Other (typically much older instruments) may have the simple (or "Albert") system. The key system is essential information to a buyer. If you don't know how to identify them see the pictures in my other guide or take plenty of photo's so that buyers can clearly see.
Pads & corks. Are they all intact? Are the pads heavily stained or new looking? A good way to check pads are sealing on a clarinet is (seperately on each joint) cover the holes & press down the keys with one hand whilst putting your other hand over the bottom of the joint. Then cover the other end of the joint with your mouth & blow (not too hard but more than just breathing out). If you can't blow then the pads are sealing. If you can then air is escaping somewhere & one or more of the pads needs attention. Has the instrument been serviced/had its pads replaced & when? State this in the listing.
Does the instrument play? If you're not a musician, try to grab hold of a friend that is who can play test it for you.
Age. Of interest but not crucially important. A well looked after & regularly serviced instrument (especially a pro model) can be just as good as a new one.
Reeds. Used reeds are of absolutely NO interest to any buyer. Throw them away. The only time reeds are a selling point is if they are brand new & (preferably) still sealed in their box.
Mouthpiece. A word of caution here. In many cases the mouthpiece will NOT be from the same manufacturer as the instrument. Musicians tend to buy mouthpieces seperately so just because the mouthpiece has something stamped on it doesn't mean the instrument is of the same brand. Many musicians will look to add their own to your instrument but in any case, list & photograph the condition of the one you're supplying. Is the tip intact or chipped? Are there gouges (bite marks) on the top face? Does it include a reed guard/cap? Is there a ligature - the thing that straps the reed to the mouthpiece?
Other accessories? The case - over-all condition, are the hinges OK, do the catches work? Pull-throughs, pad savers, cork grease, slings. Are they included? They're only cheap items but let your buyers know whether they're getting them.
Reason for selling/any other background info. To a musician an instrument can be an extremely personal thing. Trust me - buyers are interested!
Packaging. Buyers will not mind paying an extra quid or so for a decent amount of bubble wrap.
Good luck and happy selling!