It is more by actions than an actual contract. If you make your way in the world by lying, stealing, killing, etc., with no concern for anyone but yourself you have in essence "sold your soul to the devil". If you follow God's teachings you won't do those things and can go to heaven instead of hell.
No! One might truly believe that they did, but by accepting Jesus into your life that contract is broken. you can make a decision to go your own way and end up serving the devil by doing so. Jesus already paid a price for your soul but it's up to you to receive it.
Well of course! Satan, no matter how far away he is imprisoned, will ALWAYS find a way to mess with humanity. Whether it's a demon or some aspect of Satan himself, there will always be evil on Earth. Whether or not that evil will try to mess with YOU, I don't know.
Yes. Most people are not depraved enough to do it for themselves. Check out ghost rider, the movie, he did it too save his father. And, you don't really sell your soul, for some reason, satan don't want that kind of deal. You make a contract, an agreement, for what you want, and what he wants. This is where it gets ugly. Satan waits till a point of extremely high pressure for you, then promises your dream. You agree to an open contract, his side is not declared. He comes in his own time to collect, whatever he wants. It will never be your soul. Your own deeds will get you to hell. He had no reason to bargain for that. Well, I guess now that I've worded this as such, my answer is no.