I agree that from that post is sounds like you really love him. You could take it as a sign that you are meant to keep him if you keep having problems selling him. If it is money you are worried about have you considered leasing? If the girl can't afford to buy him, could she afford a half lease? That would give you money, but still let you have full rights to him. Sometimes the horses we make plans for never come to pass. I never planned on owning an Arab. I wanted a big beautiful TB. I LOVE tbs. But I worked with this Arab for two years and when it came down to someone else possibly buying her and taking her, I freaked out and flat out bought her. I never looked back.
Ask yourself: if you lost him to someone else, if you woke up tomorrow and you didn't have him, how would you feel? Would you get over it quickly? Would you think about him now and then and move on? Or would you just lay there and cry? Be honest with yourself because that will give you an honest answer as to if you should keep him for sale, or just keep him! :) Good luck!
~He knows when you're happy
~He knows when you're comfortable
~And he always knows when you have carrots.