I am a professional bone player and an avid Ebay buyer. I often see horns listed with little or no info and I'm sure the seller wonders why they have few or zero bids. Most buyers are looking for specific models or attributes and need this info to bid intelligently.

1. The piece of information buyers need the most is the model number. Knowing the model No. will usually tell a buyer the bell size, bore size and playing level the horn is designed for.(Student, Step-Up, Pro) You can find it on Conn horns stamped on the receiver or connection between the bell and slide. Others engrave or stamp it on the bell. Some models are hard to find info on so if you know bore and bell size please list it.
2. Measure the bell diameter. If you can't find the model number this helps determine the model.
3. Listing the serial number will assist in finding when the horn was made. Some sellers list only the first few numbers to prevent someone trying to scam them. I think this is an acceptable practice.
4. Don't list the length, they're all the same. (except alto and contrabass)
5. You don't need more than one picture of the case.
6. When you take pictures of the horn get as close as you can and still be in focus. Have the room well lighted, ideally  well enough you don't need a flash. The flash tends to reflect from just what you are trying to show. Get close ups of damage or special items like a broken valve trigger or braces that need soldering. A shot of the whole inner slide will assure  buyers there is no damage or wear.
7. Don't lay the assembled horn with the slide flat and the tuning slide on the table or floor. This creates a springing force on the slide and can warp it. Turn it upside down in a three point stance with the bell, mouthpiece and tunning slide. The better the horn, the more important this is.
8. Having an F attachment (trigger) does not mean it's a Bass trombone, or a valve trombone.A Valve Trombone has three valves like a trumpet, no slide. Having a bore size of .560 or larger makes it a bass trombone. That is why the model number is important to buyers. 
9. Mouthpieces are never solid silver or gold. They are silver or gold plated brass.