Join Date: Sep 2012
Location: Central Western NSW, Australia
You really just need the facts and good quality photos, preferably conformation shots.
Don't go rambling about your horse, and don't use fluffy words. Tell it as it is - the more straightforward the better. You don't need your horses' full story. Personally, I would include breed, age, height, rider experience required, riding disciplines, and how they are to catch, float, shoe, etc. You do want to include a little on personality and quirks if it is important - for example if they are docile and can handle just about anything you can throw at them, or if they can't be kept with mares or whatever.
Keep it brief and make sure it is easy to read, aka don't pile it all into one paragraph. Try to think of what you would want to know if you were buying. The more essential information you include, the more genuine inquiries you will get, rather than heaps from people who are just looking for a tad more information.