If you were honest in the add, I think the biggest thing is, as was said, be completely honest, but focus on the good. I was at a sellers who was really honest, but focused on the negitive traits. I had the ability to fix the things he was so negitive about, and I really liked the look of the horse and the price was right, so I bought her. He focused on how hard she was to catch, but the first time I went to catch her she ran up to me and put her head in the halter. His way off selling would have turned many buyers off unnessessarily.
Be honest but possitive, with the goal of finding a home where both your horse and the new owners are happy.