i just signed up for this a few days ago as well, so maybe i can explain as a newbie better than a seasoned vet like kosmo. basically, once you have something on your "want" list, if somebody has that CD on their "have" list, they have the option is shipping it to you the second you put it on your "want" list. so don't put something your "want" list that you don't want, because you might get it. on the other hand, just because something is on your "have" list doesn't mean you have to ship it. you can bug people by messaging them, but it doesn't seem too necessary.
you have to build up credits to get stuff shipped to you, i think the first one is free but after that it's kind of a one-for-one thing.
that make any sense? i can't believe i got someone to take my copy of "pretty hate machine" already! and i got "thank you" by royal trux out of it, what a deal!