Dream horse.com is a good one. Also depending what breed you are looking for go to that breed's web site and they will have classified ads on horses for sale. Good luck...
I've bought my full-size horses either locally or raised them myself, but I've looked on Dreamhorse.com, FreeHorseAds.com, Hoofandtails.com and the ones you mentioned.
I've been contemplating selling my breeding stock APHA mare, so will be looking into those sites soon myself.
I found a mare I want from a rescue...perfect in every way, except one...she is six and a half hours away, but they would meet me half way...ugh...hubby is gonna shoot me dead!