1 - A middleman is somebody who assists in making a trade safer. They are not required but they are suggested if you are new and/or are unsure about the trading process. They are free.
2 - You are able to make a trade about selling/trading/giving away, etc just about anything. Just be sure to use the right section.
Try this thread for starters: (you need an account to see links)!
3 - You can post a new thread but I'm pretty sure most people who "trade" check that thread fairly frequently.
If you have any other questions, feel free to ask away!