Measure your round pen, measure your horse then come back with that info. If your horse is really doing what you say he is, then he is probably of some value to someone in the jumper industry. I have a feeling you are being advised to ride with out a bit to preserve your horses mouth not because most people start without a bit in their mouth. Also if you meant 15 and 1/2 hands, then that is 15.2 not 15.5. A hand is four inches, so a 15.2 is a 15 hands and 2 inches, this is how we measure horses, don't make the mistake again, you appear ignorant and now everyone is thinking what is someone who doesn't know how to measure a horse doing riding an unbroke one?!? It seems to me you don't have a firm grasp on the terminology, calling your horse green broke when it's barely started is another thing that makes me wonder about your ability as a rider. Based on what you have said you need professional advice, so pay a professional to come and work with you and your horse, or asses your horses suitability to you and to jumping. If the trainer advises that this is not a suitable horse for you but could be sold as a jumping prospect to someone with the proper knowledge then maybe you could take your profits and invest in something with more training. I have a feeling the people around you are well meaning in their advice but don't have the time to dole out free advice to someone with quite such a project. I used to meet people who said they could ride, but then they would come to the barn and it would turn into me babysitting them or giving a free lesson. I don't end up riding with them very often, instead I stick to my horsey buddies that no what bridle to grab if I tell them what bit is on it. This sounds like a lot of work that only an advanced rider should tackle. Also if the horse is jumping out of pastures then was he really jumping out of the round pen because he was scarred or just because he can? Either way, a horse that can jump that big, no matter what the breed or height, is worth something, especially since it sounds like he is so willing to do it.