You can list them on EBay... You can also search Yahoo Groups for horse ones too... You can search the net for the online horse selling sites also...
Have you thought of leasing them? - I have been told feed/hay is high... People can't afford to keep their horses... (job situation too) - Years ago horses were leased to 4-H'ers and alike a year at a time...
You could post their pics in www.FOTKI.com or www.Flickr.com for free to sell them... Post link in your signiture...
Create blogs on them too...Post link in your signature...
My 1st horse was an American Saddlebred... I love the breed... Rebecca
You are not allowed to sell live animals on ebay. Tack, equipment, and I believe even stud service is allowed, but not the animal itself. Try dreamhorse.com. That site gets more hits than any other on the internet that I have found. Cost $20, your ad stays on for 3 months, and you get to put a picture on with it. Be sure it is a good pic, not one of the "dumb" ones with the horse standing down hill, head hanging or looking goofy...like mine all do anytime you try for a nice picture! :)