Melanie, you are selling Eventing photographs and have built a forum designed to allow other amateur photographers to sell to Event riders, although I suspect that it's mainly to promote your own work.
Official photographers are the only ones who are permitted to sell their images from BE affiliated horse trials to non media sources without the organiser's express permission. You are also in potential breach of British Eventing's intellectual property rights, and I suspect that the same would apply to BSJA and BD affiliated events. If you encourage anyone to sell images from any racing run under rules you will run into the BHA fairly quickly.
I can understand enjoying equestrian photography, but can I suggest that you either continue with your hobby for pleasure only, or work your way into the business like everybody else has to?