Can I sell a horse that my ex-fiance bought but has left on my property?

Unless you have his permission to sell the horse (preferably in writing), or you are covered under the Washington Agister's Lien (http://apps.leg.wa.gov/rcw/default.aspx?cite=60.56&full=true) no you may not sell the horse. If he is willing to sell to you it is likely he is willing to sell to someone else. Why not just ask him? You should speak with a local attorney for guidance. Equine Legal Solutions should be able to give you a referral: http://www.equinelegalsolutions.com/

What you really have here is a situation of abandonment. He left the horse behind and then gave you a deadline of the end of July to buy the horse. How much was he offering for it? Fair market value? Pennies on the dollar? An exorbitant rate no one would pay? Also, did you make any indications that you might be willing to buy it? Did you give him any money for it? Since he has not come to retrieve the horse and the deadline was nearly half a month ago, I would say he has constructively abandoned--that is, given up--all ownership interest in the horse and it is yours to do with as you wish. You can either keep it and continue caring for it at your own expense, or you can sell it to whomever you wish for whatever amount you wish. He only owes you for the cost of feeding and caring for the animal up and until you sell it if that is what you choose to do, or until the deadline passed for you to buy it since he never came around to collect his money. Best of luck!