Welcome aboard, Hayley. First of all, once you load your tradeables and your wants into Pinpics, when you get to a pin page of a pin you want, there is a link towards the bottom, "what can I trade to get this pin?" This takes you to the automated page where all "matches" of people who are trading that pin are listed assuming you have something they want. You can click the checkbox and an automated request will be sent early (about 4am, I believe) the following morning.
Courtesy dictates that you don't offer an HM for an LE 1000, for example. Often, you won't get a response from the person who receives that type of unfair request.
As far as success, every time you trade with a stranger, you're taking a risk. That being said, I've had about 4 bad experiences and over 100 good ones, so that's not too shabby. You can check out references (and leave them) on
www.dizpins.com, "Reference Center". I would advise doing this. Of course, since you're a newbie, other traders might be wary trading with YOU, since you have no references, and you might need to send your pin first, and allow them to send your new pin after receiving your old one first.
Yes, each sender pays postage and is ultimately responsible for ensuring their pin "makes it."
Best of luck, and have fun!