I've used equine.com to sell several horses and have been very happy with the number of inquirys I have received and the fact that most of the horses have sold.

It's easy to use and reasonably priced from free to a guaranteed sale or your money back, with several different options inbetween.

You can post pictures or just a written listing.  Shoppers select breed, price, location, displine.  Free horses are even listed.  They also list equipment, books, and loads of other horse related items.

Check out this site if your looking for a horses or equine related items.