I agree with this, with one addition. A horse that is a consummate professional and class act around the ground builds a reputation for himself, which can lend to him being more desirable under any price and these horses tend to sell themselves. Vets, farriers, grooms, barn hands, and workers all talk. I have one in my barn and the vets, body workers, etc always ‘fight’ over who gets to handle him. People have even seen how he ground ties and handles at shows and have offered to buy him because his manners caught their eye and then they watched him go.
I was a working student for several BNTs when I was a young adult and I remember getting some rank horses in our program – but it didn’t matter to the pros, because they had a solid competition record. Plus they weren’t the ones usually handling the horse on the ground. For a while being freakishly athletic over fences was synonymous with being “quirky” and/or awful on the ground. I still have a scar on my chest from a sale horse biting me almost 20 years ago. He was still my favorite, but I wish we (me, BNT, pros) had done better by him. I’m thankful that our collective horsemanship has grown in the last several decades and most people recognize now a “quirky” horse is really a horse in pain.
For non-competition homes, manners matter much more because the person doing the purchasing is usually looking for a connection with their horse.
In your situation I would make sure whoever comes to see him, tacks him up all themselves so they’re aware of all his behaviors. And if you’re certain it is not physical, you can disclose it as a behavioral quirk. Some horses don’t love being groomed, but in my experience a horse who is behaved poorly about it has pain somewhere. One of our lease horses started to really pick up his work last summer and started gradually becoming worse to tack up. He would threaten to kick especially if you were trying to curry mud off of him. In his case, it was two things: his leaser was using a hard curry comb, and he had lingering back soreness from his hind feet needing more protection with the increased work. We put him in shoes, swapped out the hard curry for a Tiger tongue, and he was better within days. Then just to be sure, I treated him for ulcers anyway and the behavior went away completely. Any time there is a pain component, I think ulcers are likely as a secondary problem.