Well, horses don't really sell her unless they have good skills. I learned that with Halo and Ricochet. If the horse is trained extensively, not many people would be interested.
If you want to make a new horse, be willing to commit to it. You might want to have a mare anyway

Speaking of committing...
TrainingPhew... I just needed to train Ammo up before He got back down again...
Ammunition--
12 credits x 4 = 48 credits added in Dressage.
Total skills in dressage -- 118
And I spend all of my credits on Ammo again...