Blind in one eye is not a significant impediment to a horse. It often isn’t apparent even under saddle or jumping.
Someone can correct this information, but I believe the USEF permits horses with one-eyed vision in jumping classes up to 3’6", maybe higher. There used to be at least one 3* event horse that was one-eyed.
Horses view the world primarily through monocular vision in any case, because of the way their eyes are located on their head. They have only a narrow range of binocular vision directly in front of and behind them. They move their long necks and probably use their keen hearing to help compensate for loss of vision in one eye.
It would be easy for even a vet to miss one-eyed vision in a horse unless they specifically did a sight test. If there was nothing showing physically that signaled a possible issue.
If the only problem is having vision in only one eye, that won’t cause a horse to tilt its head. (Being in pain from an injury might cause that behavior, though.) And if there wasn’t a cosmetic issue, it probably it wouldn’t have affected a show career. Depending on all circumstances, of course.
So all that tells nothing about what happened in the aforementioned sale. It’s simply not a significant fact that no one noticed that a horse had vision in only one eye.