You can actually get horse appraisals done, but I'm assuming it probably cost a pretty penny, and would more be something you do with either a top level horse, competing at an international level maybe, or one that has the breeding to maybe go that far, but is young, and not quite there yet. I could be totally wrong though. You should google horse appraisers, and see if there are any in your area. I know there are a couple out here in Southern CA. I agree that I have a hard time believing that someone will pay 3k for a trail horse, but it does happen. I agree that you should price him a little higher than maybe you think he's worth, or that someone will pay, but then put or best offer. Less likely to get tire kickers, and first time horse owners that don't know about horses, and are possibly going to be getting in over their heads. That being said, finding a great trail horse, that is sure footed, doesn't spook, goes through or over anything etc. is a hot commodity, as there are plenty of horses advertised as potential trail horses, but they are spooky, hate water, have few miles on trail, or whatever, and you end up having a horse that really isn't anything like what you were hoping for.
I had the same issue with my first horse, he got to the point where he just couldn't jump as high as I wanted to, as he was getting pretty old, and I had to make a choice of either just sticking with flat and trail, or selling him and getting another jumping horse. I made the choice to sell him, because I knew that I wouldn't be happy just doing flat, and it wasn't fair to him to keep asking him to do something that he couldn't do anymore. Found him a great home with a young girl who loved him. Hard choice, but in the end I know I made the right one for both of us. Good luck!