So far, once I've resolved to sell a horse, it's sold quickly and I don't think ever longer than 1 month and I don't have any horses that aren't quality.
What I do is before I adveritise, I make sure I have ALL the conformations shots including profile, bite, legs. I also have plenty of casual shots.
A couple times, I've put a horse on my own sales page figuring I will get serious and advertise in "a few weeks" after I've gotten my pictures all lined up and sure enough... a couple days after I added the horse to my sales page, I've gotten inquiries and then scramble to get the pics taken.
Another thing I've learned in my actual business (ie my day job) is don't appear desperate. Even if you really want to do a sale (or get a new client), be calm and cool about it and it's more likely to happen.
I really have to credit Lil Beginnings with my ability to sell horses quickly so far. I've been running banner ads on this site monthly for a couple years and do really think it pays off. Also, I think the help I've had with a logo that people now recognize has been another factor. These things I think have helped build my "brand" as it were.
Good luck!