Listing horses for sale. 
Assuming you have permission to sell a horse (e.g. you own it, you’ve had it for 6 months/3 uses, etc), you will see an Offer for Sale button on the right sidebar of the horse page. That will give you options to sell the horse.
Once a horse is listed for sale, you can view it on the sale page, and edit/delete its sale offer from its page.