I bought both of my current horses (several years apart) through the same person (Martin Videla), down in Argentina. There the seller pays commission to the finder, so even though in both cases he helped me find suitable horses, had me try them the first time, when I was able to go to Argentina, and in the second case sent me endless videos of horses for sale that he thought might suit me, set up the vetting for me, steered me to the right people to get them both imported, and so on, his commission (10%, which I believe is the norm down there), came entirely from the seller side.
It turned out that, although I looked at many, many other sellers’ horses, in both cases I bought from Martin. The first horse was from his Rancho Pampa operation, the second he co-owned with someone else, so he took whatever commission out of the transaction he was owed, both times. Everything was 100% transparent, and even if I had bought from a different seller it would have been exactly the same- I paid zero commission, and the horses’ prices were all known by everyone up front, and whatever padding went on it was worked into those prices (which were very, VERY fair).
On my side of things I worked with my two trainers who looked at the videos of my top choices and gave me their professional opinion. They were both eventers and also friends, and things are a bit different in that world than H/J-land. When I asked what I owed them for this help the answer from both was “Nothing.” From their perspective I was a client who was now bringing in another horse, which equalled more training and more lessons and so on. Somehow this model seems to work just fine, so I would argue that it doesn’t have to be the way it is in H/J-land.
I grew up in A show H/J-land, so I learned early on about commissions from our experiences buying three junior hunters- the lack of transparency and the gouging in this process sucked then, and it sucks now. I compete in the jumpers still but this sort of practice is one reason I no longer board or train with a “normal” H/J show barn. 