Usually the buyer pays a commision for help in finding a horse.
If I was selling I wouldn't pay the buyer's trainer anything but I would pay my trainer to sell the horse for me.
Basically you pay for the time & knowledge of whoever is helping you but it shouldn't be the same person for the buyer & seller.
Usually they get a percentage of the price of the horse. Not sure what price range you are in but standard is 10% many times you pay that and or they get a kick back from the seller :)