Having gone through the research and shopping process for my first (endurance) horse less than a year ago, I have to tell you that I would have never taken an ad for a QH seriously at all. Maybe a QH/Arab cross, but not a PB QH, especially not one without a proven track record in endurance, or at least CTR.
I have no intention dismissing the breed at all, and just with anything in life there are exceptions and unusual success stories, but anyone who is more serious about this discipline is just so much better off with a breed that’s “made” for longer faster distances.
If this horse has great stamina, find out what his recoveries are like after 10, 20, 25 miles, and also what happens when he is asked to climb or ride in the heat of the day. What you think looks like stamina now, may not be when put to the test. And remember, “real” endurance begins at 50 miles…
So it’s probably best to check how good a trail horse he is and advertise him as a CTR prospect first. Maybe someone wants to take it on, and next thing you know, he finishes Tevis! But there are too many unknowns, so I can’t imagine you will find any serious endurance riders buying. And to be honest, an inexperienced rider may easily do damage overriding a horse that’s not suited for the sport.