I've had good luck with dreamhorse.com and equine.com. I avoid craigslist like the plague. But there's a long story behind that involving something that happened to a friend. I found my horse Dallas on Dreamhorse.com and he's been a phenomenal show horse and partner.
I like ads with pictures and video. I want to know the horse's age, height, approximate weight, show experience, training level, reason for selling, etc. The more info the better for me.
<3 Dallas, Dakota, and Shaymus <3
RIP Shaymus 8/16/13