For the most part, I think sellers believe themselves when they say the horse is sound. There is also what you said above - their definition of sound is totally different than others.
Everyone has a different metric for soundness and what threshold they will tolerate for unsoundness. Detecting lameness is a genuine skill that can take years to develop because it is, IMO, an experience-based skill with a side of analyst. It is about so much more than just the way the legs behave. Watching every piece of the horse in motion and critically analyzing the range of motion and how it compares to complementary parts of the body is difficult - and it also requires, to a certain extent, a natural “eye” that is very similar to that elusive “feel” under saddle. Some of us are born having it. Others have to chip away every day to be able to confidently see or feel it. Not only “seeing”, you need to know what that thing you saw means. A horse that does not have full range of motion in its knee is not always hurting in the knee. Sometimes they’re protecting sore heels, sometimes they are protecting their elbow, sometimes it is the result of an old DFFT injury and loss of elasticity in the limb. The place of unsoundness is not always the source of unsoundness, to the frustration of many vets and horsepeople everywhere.
Look for the lack of harmony in the body - it is sometimes the easiest way to detect a subtle lameness.
There are amateurs out there who have amazing feel and trainers who couldn’t feel their way out of a burlap sack - and it’s the same for the eye for soundness. There are horse people who have a natural eye for unsoundness and vets who need the horse to be a 3/5 before they can detect the lameness. The older I get the more I realize there are more vets out there who do not have a natural eye than vets that possess it. It’s the same for every day horse owners and trainers too.
I have been accused on this BB of thinking “every horse is unsound”. While the remark was meant to antagonize me, the poster wasn’t wrong. Most horses I see are not 100% comfortable in their own bodies, therefore, are not 100% sound - including my own. There is a grey area there, a swath of “servicably sound” horses. That horse with KS that is ridable is servicaby sound. The horse with navicular that needs coffin injections yearly is servicably sound. Etc, etc.Sellers should use this language instead, my honest opinion.