How much shall I sell my pet for?

If he is 12 years old, that is old and nobody would really want it as a dog. Most families buy dogs when they are pups, not 12. If he is hard t handle , go to your closest animal shelter and give him up there. For example; North Shore Animal League and they would love to help you in your case. For now, just give the dog love and food and shelter, that is probably all it can ask for. From the description it seems like he has had a rough past. So comfort him, maybe it will make him calm down, :)