Horses can be sold in 2 ways.
1. You can sell the horse in your shop for a price you set. (You must be level 10 to open your shop.) To do this click on your shop icon then in the menu under the section 'Sell' click '>> Horses. Check the box next to the horse you want to list for sale, type the amount in the box and click the update list button.
2. You can also set up an auction for you horse in the auctions area of the market. (Horses must be level 20 to be placed up for auction.) Use the help pages in the auction area to list the auction.