If you click on the member's name you can get more details about them. There is a section called something like 'member's community profile' - if they have a profile, that means they are on the forum and you can then send them a private message through the forum. I think that's the only way although a moderator might be able to tell you more.
Yes that is the best way, though lots of people sign up to the forum then never visit again, or just don't know where to find their private messages, and not everyone gets the email notification of new messages. Another way of doing it is to accept the swap which will then give you access to their email address via the More Details link for that swap. If you email them straightaway to explain you'll be happy to send the book for postage, then it may all work out as you intend - but of course you run some risk that they won't see your email before they send out their book (sometimes emails from members end up in people's spam folders), in which case you'll end up with a book you don't want, but you could always try and swap it on... or take it to the charity shop! And I should add that some members can be a bit suspicious of an offer to send a book for postage only, perhaps assuming they'll be asked to send more than the actual posting cost, so when contacting them it might be a good idea if you can state the weight and posting cost up front.