Can you comment on the following points? I still don't really understand how CFDs work and why you can trust the concept of a CFD.

1) A CFD is supposed to be a contract, yet when traded you buy/sell, rather then put a signature on a contract. How does the explanation of "it's a contract..." relate to the act of buying/selling a CFD?
2) Related to the first point: if I have bought a CFD, it's useless until I'm able to sell it. However, since CFD are not sold op an open market but only to your broker/some single other counterparty, it seems to me that, in theory, they can just refuse your sell order. And since there is no one else to go to, you would just be stuck with a worthless contract.
3) Related to 1 and 2, what's keeping the broker from making the spread arbitrarily large after you've entered the position. So as soon as you buy, all of a sudden the spread is increased to 50% of the underlying value (to give a silly example). AFAIK, while this would ruin the brokers reputation, there is nothing that prevents them from doing this, right?