Not in any particular order but agree with what has already been posted.
Especially on these ad sites, don't list the price as $1 or for that matter as private treaty; listing a general number like mid five figures is OK I guess but I would still prefer to see an actual number as "mid-5 figures" could indicate anything from $45K - $65K and $20K is a VERY large gap. It wastes my time and the seller's if I have to call on a horse, get into a very long winded conversation about the fact the horse walks on water, only to find out the horse is more than well over my price range.
Pics are preferred and these should be correct conformation shots. I don't need to see the horse eating grass or paling around with his/her pasture buddies. As with what another poster said, "call or e-mail for vid" is fine but HAVE the vid available and don't come back with, well, I posted the ad but I haven't had time to get the video and give a list of excuses as to why a vid won't be forthcoming for another two weeks..(and that is no exaggeration on my last horse hunt).
I want a general temperament evaluation; level of rider required; ground manner evaluation; training; show history (in general). Was the horse shown only at schooling shows or were they shown at recognized shows?
I admit I found my last horse off a very basic ad that had no picture, no video, and extremely basic information. Normally I would have passed but something kept bringing me back to the ad. In one of the conversations with the owner as I was going through the PPE process, she asked me what I thought of the ad. I told her it would normally have been one I ignored and she said she writes the ad on what she tends to look for. To her, getting all the other info first hand by phone, not even necessarily by e-mail, is the important part. She did indicate that several of her friends had pointed out that times have changed and people expect photos and video :)
As for the video, if someone is selling a horse as a dressage horse, don't post a vid of the horse jumping, free jumping or being ridden in a lazy type manner. One vid I found was in an ad selling a dressage horse that never showed the horse's movement. All it showed was the horse being groomed and tacked..and they took 15 mins to do that. Also, the vid shown should be current. One horse I looked at had a great vid...from three years ago, but, the current physical shape and condition was vastly different and didn't resemble the vid posted for the sale ad other than the horse was the same color. The asking price was fine for the vid placed but the asking price for the horse when I saw him put the asking price as way overpriced..by better than half.
And sorry..but I agree. Spelling, grammar, ad accuracy and general flow is also important. Don't repeat information, don't copy and paste someone else's ad (called on a gelding only to find out the horse was actually a mare..ad was copied and pasted from another horse's). Same with another gelding..the name in the ad (not really important but the seller never did correct the ad) was of her stallion and of the horse for sale..the age and foaling dates were all wrong as well. She had her stallion's age but this horse's foaling date and the age/date didn't match.
Don't post an ad titled with "Great personality and talented horse for sale, call for price and info" with absolutely no other information. Almost guaranteed the seller isn't going to get too many calls.