I agree with above posts. Research other peoples ads in your are, you don't want to be low and need to ask more (I don't think that works so well..lol). And you don't want to be too high and not sell him. I live in Alberta Canada and I find horse prices are very different here than in the US.
In the ad I would let people know that he's a friendly horse that needs some fine tunning or more miles or something along those lines. I think being upfront in the ad will help bring in the right buyers. I know I wouldn't like it if the ad would say ' great horse' then I go look and here's some issues that I didn't want and my time and yours was wasted. Good luck.