First you should reach a deal by chatting with the other party. Then you will find in the right side of the text box a button called "Agreement". Touch it and check your switch (the first party to do it must select a date for the sale) and wait until the other party confirms. Once you meet in person to execute the sale, both of you must check the delivery switches, so that Goby knows that the transactions has ended. You will both receive a feedback link in the chat conversation in order to assign the other user the rating you consider appropriate.
Easy. In the search screen, write in "What do you want" what you are looking for and in the "What do you offer" the item you would give in exchange, then hit the search button. You can also check the details of any offer to see all items that the seller is interested in for an exchange.
Of course, but only as long as you use it with your parents or legal guardian's consent.
Share the App, and specially invite your friends per email or SMS to use it, or tell them in person.