Aging horses are sometimes hard to give away much less sell. People don't want them for the same reasons you want to sell them. Like EZ suggested maybe a therapeutic center. Another would be to contact a 4H group. Someone just getting into horses might be interested IF the horse has good ground manners and is sound for light riding.
Karen ~ Trails & Joe Paint Gelding Paoli, IN
"My treasures do not sparkle or glitter, they shine in the sun and neigh in the night."