Question regarding contracts

I have a question regarding the way the contracts work. If I sell contracts does someone have to buy them before I stand to make or lose money and do I have to buy someone else's contracts that they have put up for sale if I want to buy or can I set my own buy price and if I can set my own by price does someone then have to sell at that price before I stand to make or lose money?

Any time you complete a sell transaction, some other member is buying these shares. The exchange neither buys nor sells shares.

If you shortsell a contract (you sell shares you didn't previously own), you funds are frozen for the full amount if that contract went to 100 (a losing contract for a seller).

You can set your own price for both buy and sells for a contract. Some other member must agree to your price and buy/sell for those contracts to become valid.
Lets give an example:
You want to buy the Auto.Bailout.Dec09 contract. But you don't want to pay 20$ (which is 2$ per share at 10 shares). So, you put in a Limit buy order for 10 contracts at 15$. If/when the price moves in your direction and someone is willing to sell shares at that price, your shares will be picked up and you will see that contract now listed at the bottom of your 'Trades' tab.
You do not have to wait for the contract to expire to make a profit. If it goes higher you can sell at any comfortable margin. Many traders never wait for it to expire and lock in a profit at some point. At times it can be quicker to take the money out with that profit earned and reinvest elsewhere.... such as the dangerous DOW dailys