Well, To answer your questions.
1. First of all it is hard to sell a car which is financed because a buyer wants a title when he/she hands over the cash. You could sell it to a person that you know who is willing to wait for the title....Usually banks send title in few weeks after it is paid for. Now if the buyer is writing a personal check, he/she can write it directly to the bank with the account number and once it is clear, the bank will send you the title.
2. You have to call the bank and find out your payoff and if the selling price is more than the amount that you owe. You could then have the buyer write two checks, one for the bank for the payoff and the other one to you for the difference.
3. No, the bank will not release a title unless the car is paid in full. So if the car is sold for less than what you owe, you MUST have the difference to pay the bank. If you don't have the difference to pay the bank and the buyer has already paid you the amount then you're in trouble because the buyer will not receive the title.
4. If you give the car back to the bank. It is called as voluntary reposession, which could seriously hurt your credit. You could go to a dealership to see if they'll buy it.
Hope this helps.