I might add that the reason people use agents in private sales and buys is time. Most owners don't have the time or the know how to watch a zillion races a day, nor do they want to. They figure if they hire someone to do it, that its worth a small percent.
By the same token agenst are often used to sell horses, especially brrodmare and sire prospects. Owners simply have too many other things going on in their lives to do that. it involves making a lot of phone calls and contracts and haggling, etc. So they just tell someone how much they want for a horse and let them do what they consider to be nuisance work for a percentage.