If I were to be selling a horse, I make sure the horse is clean,(not to the point of going to a show) but no mud, few stains if a white/gray horse, mane/tail combed nicely. Not to overdo it to make it look like your trying to over impress everyone.
Conformation pictures, then maybe one or two of either movement or a fun pic.
For a video, nothing drawn out, short and sweet. A bit of leading the horse, then riding, 2 min at a walk both left and right, then without cliping the video get into a trot, then to lope. Show the stop and backing up of horse.
One last thing, keep the distance of the horse and camera close! It is one of my pet peeves when you can barely see the horse because it is to far from the camera.
Simple, and to the point that's my motto lol
~The Almighty put hoofs on wind, and a bridle on lightning, He then called it horse.~